Facebook shows comments from your friends highlighted in your feed.
So if one of your friends comments with the one millionth comment, you can end up seeing the post and your friend’s comment In your feed. So while no one can read all comments - your friends are likely to see yours.
So it's actually sharing with added notes, like quote tweets. Indeed makes a bit more sense now, with this piece of knowledge.
(Perhaps once upon a time I'll end up also finally figuring out how the hell comments on Tumblr work. Not that I'll have practical use for that anymore.)
I've seen a lot of this recently, both on the types of posts in the article and on the posts of controversial / extremist right-wing politicians like MTG.
So if one of your friends comments with the one millionth comment, you can end up seeing the post and your friend’s comment In your feed. So while no one can read all comments - your friends are likely to see yours.