I’m not the GP, but the reason I capitalize words instead of italicizing them is because the italics don’t look italic enough to convey emphasis. I get the feeling that that may be because HN wants to downplay emphasis in general, which if true is a bad goal that I oppose.

Also, those guidelines were written in the 2000s in a much different context and haven’t really evolved with the times. They seem out of date today, many of us just don’t consider them that relevant.