It's the complete opposite. The objective of type hints is that they're optional precisely because type hints narrow the functionality of the language. And evidenced by the fact that different type checks have different heuristics for determining what is a valid typed program and what isn't, it seems that the decision is correct.
No type system will allow for the dynamism that Python supports. It's not a question of how you annotate types, it's about how you resolve types.
Nobody is saying they are mandatory, and I'm actually a big fan of gradual typing. My point is that they do nothing.
However, type hints reducing the functionality of the language isn't true either.
Optional on paper, sure. Once you publish shared libs or keep a nontrivial repo usable across teams, type hints stop feeling optional fast, because the minute mypy, pyright, and Pyre disagree on metaprogramming or runtime patching you get three incompatible stories about the same program and a pile of contraditions instead of signal. Python can stay dynamic, yet this setup mostly buys busywork for CI and false confidence for humans.