Yes it would make more sense to board J through L1 and Y through L2, starting with BG1.
The method stated in the OP is an inconvenience to Plat and Gold Y pax, but not to anyone else, right?
I applaud their attempt to use two doors, though. Unfortunately I think the main bottleneck is in the aisles, not at the door(s), so not sure it really speeds things up compared to just boarding everyone through L2.