Ya, there's no evidence of this. There's some hand-wavy quote in the last sentence saying it is happening, but you can't just switch in a week. This isn't like swapping a CSS file on a webpage.
To say the least. But it's nonetheless a direct quote from Epic Games' lawyer Katherine Forrest. I agree it's a dubious claim, but that's what Epic told the court.
“It will no longer be a usable engine,” Forrest said. “Developers are fleeing the Unreal Engine. It’s happening now. It’s not speculative.”
An improved title might be "Lawyer claims developers are fleeing the Unreal Engine".
Having practiced law for a decade, I can assure you that many (though not most) attorneys would say this if even just 2 or 3 developers had stopped using Unreal since this fiasco started. Even if their departure had nothing to do with the current restrictions Apple has put in place. People are awful.