Probably because it is invite only
Nope. Open to public and fairly well known.
they don’t encourage anything besides listed on the site
They do. Their homepage says:
We need you! Here’s how to get involved
…
Join the r/PrivacyGuides Subreddit
Follow @Privacy_Guides on Twitter
Contribute to this website [LINKS TO GITHUB}
Chat with us on Matrix
Don’t see Mastodon listed there.
Matrix
what’s wrong with it
This blog piece is kinda bloated, who knows if it’s BS or accurate, but it popped up for: xmpp versus matrix. The privacy of metadata is in doubt, in practice it is more centralized than you might think, and more: Matrix vs. XMPP | Luke's Webpage
FWIW, of our friendly local “curators” here, I’m fairly sure @Licaon_Kter prefers xmpp, but @hans likes matrix. F-Droid discord server? - #27 by Licaon_Kter
I stand with @Licaon_Kter on this one. 