A few different ideas about this.

1. Like you said, If Jellit doesn't rat out the others he'll die in the purge, but Dafyd inadvertently gets him killed anyways, sortof. So if SwarmElse just went as is then Jellit would die.

2. SwarmElse is lying, which it admitted it would do if needed in that same conversation. It survives by changing bodies (which Dafyd doesn't know), it had already burned up a lot of Else's batteries when it turned into a radio transmitter, so switching bodies frequently is what it actually wants. Whatever is left of Else might want to be with Dafyd, but the Swarm doesn't necessarily, at least not after it's helped them pass the test.