Chess engine ratings are not immediately comparable to human ratings as they are extracted from different pools. Hydra played relatively few games so its rating estimation was somewhat approximate but it was clearly "superhuman" (GM Adams was n°7 in the world and only scored one draw in 6 games). Today Stockfish is awarded a rating of about 3500 [0] with a typical PC hardware but this rating comes from matches between engines and not with humans.
Regarding the argument of "knowing chess", it depends on you definition. I often use this analogy. Correspondence chess is to tournament chess what the marathon is to track running. They require different skills and training but I guarantee to you that a lot of understanding is involved in correspondence chess, possibly more than in tournament chess.
Oh I assumed it required quite a bit of chess knowledge and skill. But I assume what differentiates a good from great player isn't unassisted chess ability. Basically I'm wondering how well do correspondence ratings track with unassisted ratings. It was my understand they don't track really well at the higher levels of correspondence chess.
Regarding the argument of "knowing chess", it depends on you definition. I often use this analogy. Correspondence chess is to tournament chess what the marathon is to track running. They require different skills and training but I guarantee to you that a lot of understanding is involved in correspondence chess, possibly more than in tournament chess.
[0] https://ccrl.chessdom.com/ccrl/4040/