I am 50% from Phthiotis and this is a well known event in the area (people have family lores etc) but it is also well documented.
https://www.didaktorika.gr/eadd/handle/10442/23070
With that being said, I doubt the myheritage calculator is particularly sophisticated. I did input my 23andme results in there, however and it gave me interesting results. Besides 50% from Phthiotis I am also 50% from Messenia and the mix of a northern shifted maternal side (Phthiotis) with a southern shifted peloponessian paternal side (it scores Italian&Anatolian) puts me in the dead center of mainland Greek plots. Myheritage sees me as 83,9% Greek and south Italian, 12,3% Eastern European and 3,8% Ashkenazi. With the last one being an obvious misscall (23andme is great at picking Ashkenazi ancestry and I have none) from being a med with a bit of east euro input, it seems to me that myheritage sees me as 85% pre-migration era Greek/15% early Slav. The majority of the latter almost certainly comes from my maternal side (extremely blonde/grey eyed great grandma). This agrees with both Stamatoyiannopoulos’ figures and Razib Khan’s who sees modern greek being on average about 83% bronze-iron age Greek like and 17% polish like.
I scored no Balkan which isn’t a surprise since 23andme found zero areas in Albania or Bulgaria for me (I only have a handful of supposed Albanian cousins but they appear to be partially Greek). In fact I didn’t score any Eastern/western Macedonia or any Thrace at all either. Whatever Slavic input that is it appears pretty old.