I don't think this is about the Dominion suit, at least directly, for a few reasons. For one thing, see the above. Tucker brought in more money than the suit actually cost them. For another thing, Tucker told his audience at the time that Sidney Powell appeared to be full of jive, so if this was about Dominion claiming a scalp he's actually not the most obvious choice.
I think they know there's another lawsuit coming, in which Tucker will be much more obviously culpable. Probably several. The real problem for FOX after the settlement is that their "We're just entertainment, so it's not our fault if you take us seriously" shield has crumbled. They're not going to be able to get these suits tossed anymore. Whatever's coming next is going to hurt, and this is an attempt to get ahead of it because they can no longer stop it.
And that's just the civil suit side of things. I wouldn't be surprised if FOX's lawyers are still dealing with letters from the FBI asking "What else has Kevin McCarthy given Tucker and how long has he been doing this?"
Now I'm also wondering about that big Pentagon leak now. Given Tucker's stance on the Ukraine invasion, I wouldn't be surprised if FOX's lawyers are also dealing with letters from DOD asking if that leaker or any other has been giving Tucker anything. Even if the answer is no, that's a headache they'd rather not have to deal with. If a leaker were to give him information potentially damaging to Ukraine, we'd have to worry about what he'd do with it.