That's exactly why I wrote "anything in the code itself". Yes, they made a mass commit adding that attribution to every file 9 days ago to conclude a messy lawsuit. That doesn't make the two codebases themselves appear any more related in isolation.

If there's GPL violation and code was copied, there should be some traces of that. Is it truly so unreasonable to ask what those traces were?

"To conclude a messy lawsuit"? They're the ones who filed the lawsuit!

I hadn't heard about this case before, and so I don't know the details, but just from your own description, filing a preemptive lawsuit in response to an explicit copyright demand doesn't sound unreasonable - the advantages of doing so over waiting to be sued include venue selection and, perhaps more importantly, forcing the claimant to make their case, rather than letting it turn into a frozen conflict (with the possibility of them filing DMCA notices against app stores at any time). That doesn't imply they actually wanted to go to court.

Though I did just Google it, and throwing an extortion claim on top sounds... well, like a big mess.

They didn't need to do anything to "resolve their messy lawsuit" other than to withdraw the lawsuit.

That's exactly what they eventually did. I wouldn't have, if I thought a copyright claim against me were truly bogus. Maybe they decided it wasn't bogus.

Oh, sorry, I misunderstood.