That's not astroturfing/ideological manipulation, and it's not strictly evidence of voting manipulation either: people do legit like meme T-shirts and want to buy them (no, I am not in that category).

Regardless, voting is just one check on bad behavior. The other one is moderation, and at least on the subreddits I visit, moderators are strict on financial self promotion, but it can still vary.

Even on Hacker News comments there's people talking about "hey, I'm working on a startup." and someone asks "what startup?", and they reply "here it is." with a link. I've rarely seen those exchanges get downvoted.