I don't think programming languages implement decorators in order to facilitate type checking. They implement decorators because they are a nice generic way to reuse code in many functions / classes.

Now, once you have support for decorators, they can obviously be reused for type checking, but the linked repository also contains a few more decorators such as a memoization decorator.

I think those that add type checking decorators do so because it is a convenient way to add a some common code to multiple functions. The alternative for those that add decorators is to manually add a set of type checks in the beginning of each function.

Of course, if the language supported type checking they would simply use that, but if not, this is a simple way to save some lines of code.