No, that's not true at all. Just try to create an OrderedMap that supports the same abstract interface as Go's built-in map type, or try to implement a decimal floating point type that supports the same operators as the built-in binary floating point type. It's not possible.

Whether something can technically be done and whether it is good/easy/simple/etc. are totally different conversations. I'm pretty sure you can't implement a min function that works on both strings and ints in Go by using the interface{} abstraction.