I don't disagree that there are ways and means to handle things and he didn't come out looking good in this exchange whatever the excuse.
I think there's some extra history between them from the first time they met in the tournament in the Winner's Qualification. Fedor gives a hint about it in his vlog here:
He mentions drama with rules and referees around 7:28 so...
None of that excuses his behaviour which was petty and starting to move towards verbally insulting / mocking which is the beginning of bullying. You are also probably right, there are likely people he wouldn't have done it against whether that was bigger names, people he interacts with more, or just physically bigger people!