This isn't my AMA but I'm happy to give my personal view on this. Masking is much wider and it's about not looking odd or different rather than seeming polite, although there might sometimes be overlap.
For example, you wouldn't think me rude if, at work, I suddenly started rocking back and forward and muttering the same phrase to myself at my desk, but you would think me odd.
It wouldn't be rude of me to introduce, as a topic of conversation, the differences between a mk1 and a mk2 railway carriage, but most people I know wouldn't be particularly interested (my work colleagues mostly seem to talk about their foreign holidays - I don't do foreign holidays because I can't cope with them - their children - I don't have children because I couldn't cope with them - and what they are watching on streaming TV - I don't have streaming TV because I find Freeview sufficient for my viewing needs).
You might think it rude if I didn't make eye contact when talking - in fact, I have had negative feedback at work for people not feeling I am paying attention to them when they are talking to me - but this is very difficult for me, so looking someone in the eye is another way that I mask.