Text links are a very simple button type.
Eh? I didn't know links were considered buttons. I'm not sure I totally agree understand, but it's not outrageous either...
Update: Okay, I guess when you put an outline around it (like they directly below this paragraph), and even more if you put an icon with it (like they did further down; https://hyp.is/DZTZzi6fEeuu65uvQJ9W1Q/uxdesign.cc/ui-cheat-sheets-buttons-7329ed9d6112), the link looks like more like a button.
But (and I think this is their point) it is what it is because of how it's used and not how it's styled: it should be the same thing (a button) whether or not it has an outline.