You have to remember all these tricks :

https://github.com/golang/go/wiki/SliceTricks

You can't even wrap them into functions or you'd have to write type assertions everywhere.

Now,you could define a custom Array type with methods, but goodbye type safety. It's funny how sometimes Go works better as a "dynamically typed" language than as a statically typed one.

Ruby on the other hand provides loads of methods for every single "core" type.

I do think people recognize the fact that these two are different things (e.g. when comparing C to a language with a richer standard library it's one of the points that are often specifically and separately mentioned.), but it's just not very useful to separate them explicitely in a comparison of languages from a user's point of view (as opposed to a language designer's point of view).