I did know (about the angels dancing on the hear of a pin), and most people do, but i did not get that post at allPosted by Bill Ryan (here)
It's a silly discussion. I wonder if (in Russian culture!) you're familiar with How many angels can dance on the head of a pin?
It was a serious (but silly!) discussion back in the 17th century. Now, it's used as an English-language idiom to describe a pointless and trivial debate.