1) Neither company owns any part of the PostgreSQL code / docs. We do have various proprietary tools, of course, but so do other companies.
2) The companies employ quite a few PostgreSQL hackers, but if you look at the development stats (e.g. http://rhaas.blogspot.com/2020/05/who-contributed-to-postgre...) you'll see there are many committers and developers in other companies.
That being said, I agree with the concerns expressed by Bruce in his blog post. Or more precisely - I do understand why people in the community have those concerns, but I believe (hope) it's not going to harm the community.
Percona created their own mysql distribution [2] with a number of improvements, which was quite helpful while awaiting (very long) release cycles from Oracle, for MySQL community.
1 - https://www.percona.com/software/postgresql-distribution