C++ lets you inherit from multiple classes as well. I don't see how this has anything to do with being able to add new methods to existing types.

There is an important difference: in Rust you can write a new trait, with new methods, and impl that trait for a type without having to change the existing structs/enums which comprise that type at all. You can also do this (with some restrictions, "the orphan rule") for types defined in another library.

In C++ classes, you have to be able to modify a class definition to extend it with new methods (or a new ancestor to multiply inherit from).

You can add traits (with their methods) to existing types, without modification.

C++ classes are types. Rust traits are applied to types.