As far as I understand it, this is allowing TS types to be valid JS syntax that are just ignored by vanilla JS engines. So it is intentionally identical to TS, it just won’t have any affect on the execution of your code.
For perspective, this is more or less what Python did.
Originally Mypy was supposed to be a Python superset, like TypeScript, but instead its variable annotation syntax was adopted as Python syntax. It has been an interesting journey for Python, with a lot of ups and downs. Type hints, for all their productivity and bug-safety benefits in most cases, still feel like a bolt-on feature that requires developers to change how they write code, and in many cases favors less-performant idioms and/or extra boilerplate compared to un-hinted Python. Whereas TypeScript has a lot more freedom to mess with how things work, because it's a superset that compiles to JavaScript and not actual JavaScript.
So it's somewhat surprising to see this proposal for JavaScript, because the TypeScript model always seemed better in hindsight than the Python model! But maybe it's just a "grass is greener" situation.
It's not identical as features that have a difference on the runtime behaviour / code generation such as enums are deliberately omitted. It's intended to be a large subset of TS at first. [0]
