he doesn't let you, proving that he definitely CAN compell and control Tav. This isn't even actually up for debate, the game does in fact give a concrete answer.
Well, this is not a prove, that he could mindcontrol you. It only shows, that Astarion becomes angry and says to you, you cannot go. But what Astarion says and what he does, is free to the imagination of the players. It is meant to be ambigious. But I guess, this doesn't exist in your mind. But let other people play and feel their route how they want.
How is it ambiguous? He says outright that because the netherbrain and tadpole are gone, your future is his to decide. The tadpole that we've seen definitively negate the power for a master to control their spawn. Because that tadpole isn't there, your future is his to decide. Why would that be the case if not because he can control you? He may not spell it out in specific wording, but that doesn't mean it's ambiguous. it's like the end of Disny's Tarzan, saying Clayton's death is ambiguous because he died offscreen. Yes it's ambiguous in the most technical meaning of the word, but the movie is stating what happened so clearly that the intent is clearly for the audience to understand that the man died. Some things have ambiguity to them, such as the nature of Astarion as an ascended vampire and what that means, or what part the archdevil might continue to play in his future. But unless come the epilogue you can actually be broken up with Astarion, there's clearly no ambiguity intended in that exchange.
