Interesting. Are Lithuanians claiming prominent Belorussian figures from history didn’t know they were actually Lithuanian because calling oneself “Belorussian” was just a convention at that point – they were really Lithuanians. We get that a lot. Although, to be fair, considering Lithuania has a rich medieval history, I doubt it.
For example, the Byzantines were unaware they actually fought Macedonians at Kliuch, but they called everyone to the north of them Bulgarian, so that’s where the confusion comes from. Hence Basil II should’ve been called Basil the Make-slayer. Probably even Basil the Suigenocidal because he was from the Macedonian dynasty D