This argument doesn't make a whole lot of sense because nothing about type annotations constrains Python code at all. In fact because they're designed to be introspectable they make Python even more dynamic and you can do even crazier stuff than you could before. Type checkers are working very hard to handle the weird code.

Pydantic being so fast because it's written in Rust is a good thing, you can do crazy dynamic (de-)serializations everywhere with very little performance penalty.