You seem to be refuting claims I never made. I'm simply perplexed that so many people don't know this market exists (as is indicated/suggested by the number of upvotes).

They're not 'refuting claims you never made'. They're simply responding to your comment by making their own points. Which is a completely standard way to carry discussion forward.

I knew this happens, and I upvoted. Learning details about something I knew about is useful.

What makes you think upvotes imply people don’t know this sort of thing exists?

Because Troy Davis, in his tweet, literally said "I didn't know that black-hat content marketing existed"

I think his comments here indicate that what he meant was he wasn't aware it exists "at scale" in this way, but taking the content of the linked post at face value, it's easy to come away thinking "How did he now know this existed?". It's precisely what I thought after reading his tweets, but before seeing his detailed comments here.

An upvote doesn’t necessarily imply lack of knowledge of this existing.

I am interested in why you felt the need to make the comment, though? Because it can come across jaded like, "yeah yeah, seen it all before, there's nothing new here", which is sort of like throwing cold water on it, which can reduce the odds of positive change happening.