You’re correct that it really shouldn’t be difficult.
I suspect the code isn’t quite as minimal as that, as there’s object repositioning to do. The UI treats the plates as all laid out side by side on the table. You can drag objects directly from plate to plate continuously. Which raises the horrendous possibility that the 3D coordinates are stored internally in an absolute way, not per-plate. If this is the case, reordering plates actually means scanning the object tree and repositioning every object.
That said, they must already have a routine for that, because it also happens when plates reposition after adding or removing a plate. So you’re absolutely right in the essence; this shouldn’t be much work.