Yeah; I think the distinction with runtime types among dynamic languages is that you can always emulate any other system's dynamic type system with some amount of constant syntactic overhead in any other system. E.g. you could write a CLOS system or Julia's multiple dispatch in Javascript if you wanted. It would just be extremely ugly because you would have to wrap every function and every function call (so as to intercept the native function call and substitute your own).

Nonetheless though that's "only" a constant amount of (painful) syntactic noise per call, which means that it is conceivable for a library to provide this functionality. And in fact occasionally libraries will require users to wrap all functions in order to give those functions certain "superpowers."

It is of course nicer if it integrates very nicely with the language rather than being full of wrappers everywhere so there is still a lot of value in these runtime type systems being built from the ground-up with certain features in mind.

But it's a very different world from static types, where usually if one static type system is missing features of another, no amount of emulation will get you there.