It’s mentioned in this really good Verge article about SEO. I don’t think it’s a good sign for Reddit to just allow such blatant spam and makes me think how much subtler spam is out there too.
I’ve personally noticed more a few times in old threads you’ll find a comment made months later that’s recommending a product.
Interesting reddit suspended these accounts, but never removed the ad spam. The bots literally won while being caught.