That may be. That portion of the chip is completely closed (no public specs or compiler), so I'm just speculating based on inputs and outputs to a blackbox system.

I do know that I've seen people trick the thing (VC4 on Pi2) into pushing out 4K video at about 20fps and decoding 3 h.264 videos at once. Playing a 4th video caused a lot of visual artifacts. So it has potential for more than it's usually used for. There could be a low-level assumption of 8 bits per sample, or some other constraint that we don't know about. Or it could be as simple as Broadcom not being interested in marketing the chip toward higher-end uses like the 10bit profiles. Without signing an NDA and ordering 6-7 digits worth of chips, I think we can only speculate.