What did this?

So, I was reading a post about some more crazy tax proposals in my state, and I scrolled down to check the comments.

I saw a ton of entries with "Pingback:" in the comment section. I assume this is some BH link farming program or something? For curiosity I clicked on a few entries and they took me to different blog sites that seemed very PBN like.

Has anyone seen/use this strat?

You are 100% correct, this is some form of link building, can't say if the current website the screen shot is in on this or is just being exploited as he is showing the pingbacks.

Whenever you link to someone in your article, Wordpress sends a pingback letting the blog know you are linking to them. Some of them either approve those pings, or have them completely on auto approve.

Yes, they come from some spammy link-building sites and as you see, they don't have any real value in them.