He has written, "A map value is a pointer to a runtime.hmap structure." This is what is a reference type is. "In programming language theory, a reference type is a data type that refers to an object in memory. Reference types can be thought of as pointers that are implicitly dereferenced.", source https://en.wikipedia.org/wiki/Reference_type
I do find these posts are confusing - while technically correct they're not really very informative about real world usage, and the information is spread out over several posts instead of being in one post which explains the topic in detail. Still it's not actually very important anyway for real world usage. A map behaves pretty much like a pointer (what most people think of as a reference), you can pass it around, change the contents and the original map data will be changed (which is what people care about). In his posts he's saying well if you create a map pass it to a function then zero out the variable with a local one then it won't change the variable at the caller level, which is... kind of obvious.
I'm really not sure where he's going with this or what the context is but the blog posts seem more pedantic than useful at this point.