I don't see anything wrong with being friends with former T after the 2 year wait. Hell, I've only heard a time barrier for romantic relationships, never in the context for friendships. So you are both being careful. T sounds interested in being casual friends, so the feeling is mutual--I don't see an issue?

Maybe your T meant that you and former T will always have the same power dynamic? If so, I say so what? Former students and professors become friends, and the power differential often remains.