That thesis necessitates a shallow reading of the material, imo.
ASoIAF isn't praised for its "realism" so much as the subversion of fantasy tropes that are a staple of grimdark fantasy. The "realism" is in the notion that knights aren't chivalrous and ladies have to fight for themselves, while the commoners suffer under their lords and medieval society is generally dysfunctional.
At the end of the day it's still fantasy with dragons and ice zombies pillaging fictional societies. There are absolutely allusions to real peoples, but something Martin does exceptionally well is the synthesis of his fictional cultures from various influences in real history to create things that are familiar enough while still being distinct.