Not even true: Arm, Intel, AMD, and most other hardware vendors (who are actively making an effort to support Linux on their parts) actually publish useful[^1] documentation.

edit: Also, not knocking the Qualcomm folks working on Linux here, just observing that the lack of hardware documentation doesn't exactly help reeling in contributors.

[^1]: Maybe in some cases not as useful as it could be when bringing up some OS on hardware, but certainly better than nothing