By “social credit” system, they’re referring to the fact that Piefed has a reputation and attitude system. “Reputation” is a bit like reddit - a downvote contributes negatively to it, an upvote positively. Accounts heavily downvoted will have negative warning signs on them (although the CCS here can be disabled by instance moderators who dislike it) so users can’t see it.

“Attitude” refers to downvotes given. If you mass downvote, you’ll get a very bad attitude and this is measured in percentage form for users. If you bulk downvote, and get a very low attitude - you’ll be blocked from downvoting.