Being an amateur game programmer is enough to do what Rem did with p-cup dlc. Sachi already made a game with same engine before so he should already be familiar enough to be able to continue Rem's work(when he was sick, depressed or whatever bullshit reason/excuse he came up with in the past to delay the dlc).
Eeh, I don't really think it would be
that easy.
I didn't really check the insides of Karryn's Prison, but at this point, I would expect it's a mess of a spaghetti code like most RPG Maker games (especially Patreon ones, and
especially when it's a game with a lot of "not RPG Maker" mechanics).
You'd have to rework the whole game to even detect the P-Cup mode, for example.
Maybe if the game was thought with character customisation in mind from the start (like outfit change, hairstyles, glasses...), it would be easier, but here...
I really doubt an "amateur" programmer could make actual progress on the development of the P-Cup DLC.
Now, whether or not Rem was actually depressed/sick/whatever is another matter. Who really knows in this kind of situation, really?
But let's be honest, even if it was true, Sachi could've still make progress in other ways (finishing the writing and cg of the DLC, and/or do smaller-scale updates) to pass time while Rem is "away".
But again, who knows what really happened? Maybe Sachi actually did finish his work, or maybe Sachi was also taking some rest too, or maybe Rem didn't want Sachi to touch the "programming" part.