I think the purpose of optional type hinting is that you don't have to add it everywhere all at once, not that it doesn't have to be accurate. I guess you could split hairs and say "hint" doesn't imply perfect accuracy, but... adding a language feature that can lie really seems to have a lot of downsides vs upsides; whereas at least optional has obvious migration benefits.

You could have optional type hints where the runtime would still yell at you - maybe even from just an optional flag - if you returned a string out of a function that should return an int.

Because as-is, once you have those function that says it returns an int but returns a string instead, etc, in a big codebase, your editor tooling gets really confused and it's way worse to work through than if the hints weren't there at all.

(And there are tools in Python that you can use to inspect and verify the accuracy. But those tools are also... optional... And if you start to apply them to a codebase where they weren't used, it can be very time-consuming to fix everything...)