That polygon limit (65535 triangles) is for the whole car, including all subobjects combined.
So there is no trick available by using subobjects to bypass the limit. Actually, non-moving subobjects are now combined into the main mesh, for efficiency, when the car is spawned.
In fact the editor has its own limits. You can create 32768 unique editor triangles. But each of these may be mirrored, so that could cause 65536 output triangles (which already exceeds the limit by 1).
In fact these may be double sided too so each editor triangle could create 4 output triangles. So it is quite possible for a single editor mesh to exceed the allowed triangle limit.
The normal thing for an LFS mesh is that most triangles are mirrored, except the ones that you want different on each side (like dashboard). Double sided is less common.