It's not about being backwardly compatible (by and large most mobile browsers do a sterling job of that), it's about "friendly", which is to say, something which works fairly well with common mobile devices. HN has pretty tiny fonts and very small hit areas for fat human fingers.

Mobile friendly isn't a meme, it's just a description of how well your chosen presentation of content translates into a different context of use. Context changes often require different choices about how you break up your content to throw at someone - this has been the case since the web began.