I understand what you're saying, that the situation is not completely clear-cut (especially what Broadcom is doing).
But Intel has most definitely documented their GPU registers and hardware – not just the intel open source driver:
https://01.org/linuxgraphics/documentation/driver-documentat...
Intel open sourced their driver not their GPUs.
Broadcom open sourced part of the software running on one of their GPUs, not their GPUs.