I agree that taxation being theft depends on how you define theft (and how you define taxation) but that's true of any statement that says X is a type of Y.

If the Mafia had a complex set of rules and procedures governing how they stole from you, I don't think you'd shy away from calling their activities theft. I feel the same way about the government. I don't like them. I didn't vote for them. I don't agree that they should take my money and I don't like what they spend it on. That they have laws I had no part in creating, am not aware of, and don't understand, on top of judges and lawyers I don't agree with doesn't alter the fundamental action taking place - people with guns forcing me to give them money by ultimately threats of violence.

Calling taxation theft is not really a legal statement. It's a moral statement intended to make people think about taxation. Observing that the thieves don't consider it theft according to their laws is accurate, and that would be meaningful in some contexts, but that's not the moral statement being made.