What's the point if you don't actually validate with the validation hints you've added?

It's mostly a performance related choice.

Having these checks available for development does allow you full confidence in the type safety if the entire call stack has type annotations. If you're using some code that doesn't have these annotations, you might catch problems during development, or might not. But that's just the old familiar situation, and it allows you to iterate quickly while choosing which parts are more critical.

Note that this is analogous to the design choice made by Typescript, while the designers of Atscript chose to extend Typescript with runtime assertions as well.