The added bonus is not having to deal with Sony, or other people, trying to upstream stuff that doesn’t make sense outside of the PS environment and would have questionable value to others.
GPL doesn't force people to merge back to upstream, and BSD doesn't have any special way to stop people from trying to upstream. The only difference is that Sony's PS6 OS fork would have been open source if it were bound by the GPL, which would be an objective benefit.
That's what the T14s is for, no?