Sure that would explain why they don't try for 60 FPS, but doesn't explain why there aren't more GPU stressful options enabled. Better shadows, AO, AA, AF, etc. All of which are lower than the PC counterparts and have minimal CPU requirements.
Shadows could be affected by number of players on the GPU side potentially, but AO, AA, and AF are examples of GPU features that wouldn't be impacted in any meaningful way by the addition of extra players.
If they are already at the max of what they can accomplish then that's cool. But we'll never know unless Bungie talks about it. The fact that there aren't ever any drops in graphically (not CPU) limited scenes leaves one to wonder if they were just lazy, wanted to maintain graphical parity (not forced, but voluntary decision), or if it is the best they can do.
Either way, I'm glad that they didn't go completely overboard as some developers have done.
Regards,
SB