I don’t buy it.
It’s not as bad as Gearbox complaining about Unreal (which they’ve worked on since 2005), but all the coverage I’ve seen points to self-infliction.
They’ve worked on Unity since at least 2014.
And other lower budget Unity sim games like Humankind, Timberborn and Ixion (just to name a few) run fine, and look great.
Now, maybe Unity's overhead and issues is a factor, but it’s not the only one. They messed up.