There are only two engines I'm aware of that can do what is needed for Elite Dangerous: Frontier's Cobra engine, and Space Engine. Neither would be considered a general-purpose game engine like UE5.
Think about this: the latest UE5.1 update increases its supported game area to 88,000,000km2. That sounds a lot, but it's actually only ~293Ls. VY Canis Majoris is 1,975,788,000km in diameter - 22 times UE5.1's limit. Then there is the Alpha Centauri system. There would be no hope of fitting the Hutton orbital run into UE5.1.
I agree that it appears that Frontier are having trouble developing their own engine. However, engines like UE5 are designed for small-scale first-person shooters, not space sims.
It's pretty simple, like it or not, without something like the Cobra engine Elite Dangerous could not exist. Maybe they could talk to the Space Engine guys?