Dave Chaney has written "A map value is a pointer to a runtime.hmap structure.". This is what a reference type is. "In programming language theory, a reference type is a data type that refers to an object in memory. A pointer type on the other hand refers to a memory address. Reference types can be thought of as pointers that are implicitly dereferenced.". Source https://en.wikipedia.org/wiki/Reference_type
It's just a different definition of reference types. This article I think means types which point to data rather than holding it directly. It gives an example of duplicating a map and then changing that map to change the original as well, which some people would find unexpected.
I don't find that post by dave very useful for clearing up this confusion, as it doesn't include both definitions of references and this looser definition is definitely in common usage now. Depending on the language you come from, you might have different expectations when seeing the term reference - lots of people use it for pointers, even if that is not strictly true.