So they're special denizens that somehow replace what they would normally have?

Yeah. Yaldabaoth is straight up mentioned to be the Strongest Denizen given only to natural warriors, and we see both Dirk and Caliborn having Yaldabaoth. Conversely, Kanaya, Jade and Calliope have had Echidna (Porrim likely too, but some may argue Echidna was Aranea’s Denizen? It’s somewhat ambiguous), Cetus was both Vriska’s and Rose’s, Typheus for John and Tavros, it just seems like they’re tied to the Aspect, but Caliborn and Dirk are two different Aspects entirely. If we go with the theory that Abraxas is the ‘weakest’ to Yaldabaoth’s ‘strongest’, then you also have both Jake and Karkat, Hope and Blood, with the same Denizen.

So yes, those two are special and they break the pattern set of Players of the same Aspect sharing a Denizen. There’s ALSO the possibility there’s more than one Denizen for each Aspect, so you can just make Fan Denizens and have fun with that, but canonical precedent only implies 12 + 2, just like how Classes are 12 + 2 Master Classes.

It should be noted that ‘Strongest and Weakest’ is likely a distraction. Yaldabaoth is the Demiurge, the False God, a cruel liar that created the Physical World, flawed and malevolent, the reason the world is plagued with suffering and pain. Abaraxas is the True God, enlightenment, the World of Ideas, the union of opposites. And likewise in canon, Yaldabaoth is the ‘self-proclaimed’ God of all Monsters, and his Players are Dirk- Who creates AR, a Yaldabaoth-like figure, and Caliborn, who is another Yaldabaoth-like figure. They both have strong ties to the concept of the Demiurge. On the other hand, Jake and Karkat are characters that have gone through a lot of self-doubt and retracted into their shell, but manage to be considered two of the most important character in what they accomplish- With Karkat leading through his Session, and ending up a necessary part of leading the Troll Race after the game, and Jake being the one to serve Caliborn a humiliating defeat during the Masterpiece, before AR and Dirk Lil-Cal his Soul. So Abraxas definitely isn’t arbitrarily chosen as an ‘easy mode’ for ‘weak players’.