So far, no superhybrid has ever got a hybrid before. So by precedent, we say that superhybrids can’t get hybrids. We know from the creature descriptions and from JWTG that Ludia acknowledges what a superhybrid is, and it’s just really hard to convince oneself that no superhybrid has ever got a hybrid purely by coincidence.
It’s sort of like the “rule” that you can’t fuse 2 hybrids together, or the “rule” that Uniques can’t get hybrids.
Every Unique in the game is a superhybrid, even though Ludia didn’t have to make it that way. So it seems like a conscious decision, by someone who wants to indicate that a Unique is the last creature in its line.
We often used to say that if superhybrids ever got hybrids, we could then get a rarity above Unique, because all Uniques are superhybrids. We now do have Apex rarity, so whether superhybrids will get hybrids in the future seems to be anyone’s guess.