They just met them there scattered. For instance E-V13 along with J2b2-L283 is way lower in Slovenia/Croatia/Bosnia where we see a peak on Slavic ancestry. And J2b2-L283 from the data so far was like I2a2 of todays Dalmatia, but probably a lot of them after the Pannnonian-Dalmatian revolt ended up as slaves in South-Italy while others were relocated somewhere else.
I think that E-V13 is now higher in some former J-L283 core territories is obviously not fully, but partly explained by Germanics and especially Slavs coming in and bringing it from the Carpathian sphere.
Slovenia, imho, is probably not such a strong argument, because the Slovenians seem to have taken a different route, having experienced different contacts and founder effects. Like:
If going by a similar model, its obviously rather the Eastern route groups (Bulgarians, Serbs) which took up more E-V13 at home and on the way.
Different model/map, same result with the Slovenian Slavic ancestors coming from groups with the least early contacts to Dacians/Daco-Sarmatians, Eastern Pannonians-Danubians:
Therefore I think the early Slavs coming to Slovenia had less and those going to Bulgaria and Serbia had more from the start. Not the bulk, that might still be local, but it caused a major difference already. This would also be in line with a low but noticeable presence of E-V13 in practically all Slavs and East Slavs included, and not all can be explained by later Vlach contacts by any strech of imagination.
But with more data we will see how much overlap there is, between all the modern branches, not just by ancient DNA.
It's also quite telling that E-V13 has its potential "first Slavic uptick" with I-CTS10936, not with R-M458! So they were in the R-Z280 + I-CTS10936 expansion. The first R-M458 plus other R-Z280 branches expansion of the Slavs happened earlier and independently from that.
This clearly points to an Eastern-Southern going route, not the Western one. And E-V13 is in synchrony with this. Its only a limited uptick, but quite significant nevertheless. Its also the biggest uptick since the Roman conquest era.