You'd have to ask a member of the AC2 team to know for sure. I can only offer my post-fact interpretation.
As with the greater story goals, creation of creatures in AC2 often came down to, "That's the most interesting-looking concept we have. Make up a story for it." Which is great! That's how all the monsters from AC1 were designed. You can get great results when you let artists explore on their own. I remember in particular the wonderfully bizarre creations of Sonny Liew, who's since made a career in comics. I was fascinated by his revolting mucor.
An infinite amount of weirdness could be swept under the rug of the Fifth Sending between the two AC games. The spawn was a design that looked good, and might have got in on that alone. Are they Slithis? Sure, why not? They look about right. Say they're very young ones , mutated by exposure to the reality-warping powers of the Nameless, like everything else not locked in the shelters by Asheron.