It's a rule of thumb, not an absolute. This part I'm not being sarcastic abour: sorry if your social anxiety problems also include not being able to get nuance.
Oh well, boo hoo. This feels like one of those woke "Be careful what you say" speech police bullshit. Look at me, I called you "woke", I think I'm plenty left and progressive, but the whole speech policing that seems like done a lot by the left makes me roll my eyes at vast swathes of them.
On a less emotional side, dude used the term "social anxiety", he doesn't seem like he's lacking the cultural know-how. Don't be so condescending towards people... or to put it in your terms, did you ever consider not to be so condescending in your opinions of other people?
No good sentence ever started this way.
Did you consider their culture could not have the idiom? Or they could be young and inexperienced?