It is not true. That group was first mentioned in the 13th-14th century (Late Middle Ages), while the mentioned connection to scholarly invented so-called "Nabian Serbs" is part of one of the fringe theories which tried to trace their ancestry to the Early Middle Ages.
Some things other people at poreklo found:
Czech wiki:
"Velký význam však sehrálo přijetí
Nábských Srbů (Chodů) Boleslavem I. za strážce západní hranice."
Also here:
http://www.pismak.cz/print.php?data=piece&id=426346
https://digilib.phil.muni.cz/bitstr...aeologicaClassica_16-1971-1_22.pdf?sequence=1
RUDOLF TUREK
ÚZEMN Í PŘEDPOKLADY VZNIK U MORAVSKÝC H ÚDĚLNÝCH KNÍŽECTV Í, 1971
It is also no accident that the primitive shapes
of ear-rings with various trinkets found at Věstonice have some analogues in South Bohemia
(Lékařova Lhota) and also in the southern most tip of the
territory of the Serbians living along the Naab River (Matzhausen).
http://stare.luzice.cz/clv/2008/clv_2008-07-08.pdf
"Při stopování dávných severobavorských Slovanů (nejen tzv. Nábských Srbů) narážíme na potíže nejen v tom, že nejsme schopni říci nic o etnické příslušnosti a jazyce, jímž v okolí Řezna mluvili lidé pohřbení s nádobami zdobenými vlnovkou vyrytou hřebenem, ale také ve skutečnosti, že místní jména zčásti či úplně slovanského původu, resp. se složkou Windisch, Wind-, se zhusta vyskytují ve výše položených končinách, méně příznivých zemědělství, v nichž nám zcela chybějí archeologické nálezy slovanského charakteru (to se týká i našeho Chebska."
http://stare.luzice.cz/clv/2008/clv_2008-07-08.pdf
Poučná je též kapitola Sen o životě bez Němců. Zde je uváděn i požadavek na připojení části území srbských Glomačů a v české historii téměř zcela opomíjených Nábských Srbů na jih od Šumavy."
These are all Czech sources.
In Southern Croatia I-PH908 is vast majority of I-Y3120: 96/220
43,64% out of 54.5 % for I-Y3120. In Northern Croatian (Kajkavian area) sample I-PH908 is just 24/220
10,90%.
Another subclade common in Southern Croatia is I-Y56203 (dys19=14), found in Serb clan of Niksici from Tara (not to be confused with Niksici proper who are I-Y52621), as well in some East Herzegovinian samples. It has been hypothesized they might be related to Zachumlians. YF03590 at YFull is a Pole Cisowsky 1500 ybp away. 11 of these in South Croatia. 4 in the North, 7 East, 1 West, 5 in Center. I-Y56203 is under PH908 (as is I-Y52621) though it isn't yet at YFull because there are problems with reading PH908 in BigY.
Along Dalmatian coast there were Travunia, Zachumlia and Pagania. According to the first degree historical source, Constantine VII Porphyrogenitus, Narentines (Pagania), Zachumlians, Travunians are explicitly of Serbian origin descending from those Serbs who settled the area in time of Heraclius. This fits into the spike of I-PH908 in Croats from there. I-Y56203 alone makes up 5 % of Southern Croats.
The area where Portifogenitus places the settlement of Croats is of course to the West, there we see spike of R1a, especially R-Y2608 in Chakavian Croats. As you know there is no clear demarcation line between the Slovenian and Croatian Kajkavian. And Croatian Shtokavian is very similar to Serbian. Chakavian is spoken by Croats from Gradisce, Austria etc. Oldest inscriptions in Croatian are Chakavian:
Valun tablet from 11th century,
Baška tablet from 1100.
1.
Valun is on the
Cres island, where in a sample of 99 (Barac et al. 2003 - island isolates study), R1a was
56.6 %, around half were M458, half M558*. I-Y3120 is
3 % there.
2.
Baška is on the island of
Krk, there in another study (Barac et al. 2003 - Y chromosome STRs in Croatians) R1a was at
37.8 %, hg "I" (might include some I1s etc) was at
28.4 %.
These
facts give better insight as to what the original White Croatian haplogroup might be, R-Y2608 being the number one, but also there are some other branches such as R-Y132940 who likely migrated with them.
I know it must be convenient for many Croats to lump some Narentine I-Y3120's into White Croats, but Narentines were not White Croats, and Portifogenitus, who of any sources we have is the most competent to speak about their origins because he must have gained this information first hand from their chiefs, said they were of Serb origin. But I think there likely are I-Y3120 White Croatian clades too.
It sounds far-fetched. The presence of the Serbs was not brief because are mentioned there in the 7th century and according to historical sources they previously were subjects of the Franks. The refusal to connect Serbs with the Sorbs is problematic because the Lusatian tribes are mentioned only in subsequent centuries, some consider them as Serbian tribes, or Serbian assimilated, while others as separate, so either have direct or close ancestry and relationship, and that's based on historical, archaeological and linguistical evidence. The presence of PH908 is required, and not only, but I2 overall, considering the proximity and relation with Lusatian Slavs and yet that is not the case with contemporary Sorbs. The issue with the Sorbs is that they are the Slavs who have the lowest frequency of I2 (4%). That cannot be ignored, that's a fact, evidence, which must be taken into consideration regarding the regional ethnogenesis through history.
https://www.ncbi.nlm.nih.gov/pubmed/17078035
Regional study of Czechs, in Western Czech R., in Klatovy I-P37 is
14.6 %. Klatovy are one of Chodove centers.
Some of opinions I saw from some Serbs also claim Sorbs descend from tre Lusatians and Milchani tribes who belonged to a different archeological culture and only later joined the Serb tribal union. According to Sedov (and other Russian and German archeologists) Luzichani and Milchani belonged to Tornov archeological culture whereas Serbs who lived in modern Czech-German border belonged to Ryusen culture.
Yes, it's a possibilty, but considering the evidence, it does not look like a subclade or haplogroup representing "proto-Serbs" at all. It should be R1a-M458, which is found among both Sorbs and Serbs.
Now that you mention it some Serbs are R-L260 and not only that they are R-Y2905+. I took a look at Sorb R1a's and the dominant Sorb haplotype is basically certainly R-Y2905 whose TMRCA is 2000 ybp, so there might be some connection there. There is in Croats R-Y132940 cluster but they are Y2905-.
R-M458 is 4700 years old and is found in many peoples. Among Serbs M458 is less common than Z280, and under M458, R-YP417 is the most common sublcade. Then there are two clusters of R-A11460, one of whom are connected to the Slavic Berziti tribe in Central Balkans.