Hi,
It had been a while since I visited that site. Checking it now, I understand your confusion. I don’t know why they decided to re-title the site “SonarQube™ Marketplace”. It is not, never has been, and never will be the SonarQube Marketplace. That resides in-app. Period.
We’ll be taking this up with them and thanks for raising the question.
To address your specific questions:
No.
Actually I think I was fairly clear there:
We have nothing to do with that site
It’s two entirely different, and unconnected things.
No
I wouldn’t trust it. 
We actually moved that documentation into this community since it’s through the community that all communication related to these plugins happens.
Hopefully it’s clear at this point that those are two distinct things. Plugins in your in-app Marketplace were minimally vetted for behavior & user experience on initial entry into the Marketplace. After that, it only takes a passing Quality Gate on SonarCloud to add new versions.
For listings on sonarplugins.com, the site itself says
This is just a plugin compilation site.
Thx,
Ann