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.
The fact that they tried other engines seems to imply that non Flutter options were... Options.
If literally the only option was an embedded flutter view, then there was never more than one viable solution and looking into unity etc was wasted effort.
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.