This is specifically designed to embed into Flutter apps, which have specific requirements how they interact with the GPU and renderer.

They already tried other engines, such as Unity. The team didn't just go off and build something without trying existing solutions first.

Toyota complained about poor performance on all of Unity, UE and Godot, but also about long startup times with Godot.

I don't know how bloated Godot is, but AFAIK libgodot development started as part of Migeran's automotive AR HUD prototype so I'm surprised to hear it has poor startup time for a car.