Typescript and similar can do it because they don't have to specify the type system, and can't change it in meaningful ways over time. Things in the language standard cannot be easily changed, if they can be changed at all.
It's clearly not strictly necessary though. Python has shown that.
I mean I agree it is pretty mad to just say "you can write types but they mean whatever" but surprisingly in practice it seems to work ok.
The Python implementation can do whatever it wants because “Python” does not mean “The Python Language Specification”. It means the one specific implementation, and whatever that impl does is definitionally correct.
The ability for a language specification does to hand wave behaviour is very limited, and for JS is non existent (the only places where there is divergence between implementations is some squirrely edge cases of property and prototype chain mutation during for(in) enumeration).
So you can’t say “types mean whatever”, you have to specify what the implementation is required to do when it encounters those annotations. Even if they are not meant to have any semantic impact the lack of semantic impact must be specified: e.g the language specification would be required to state “here is the valid grammar for these annotations”, and specify that they are explicitly ignored and must not be evaluated or examined in any way.
