This isn't a particularly wild claim in the world of piracy. It's very, very common for folks to take high-quality media released by a group X, remove the filename suffix/nfo and then re-distribute that file elsewhere without attribution.
I will say though that from what I hear, RARGB did do some nice value-add work though. For example, once HEVC/H.265 decoding began to become widespread, and its file size savings became very clear, someone on the RARGB side of things went through many hundreds of the most popular movies and re-encoded existing high-quality releases into very tiny (and thus very popular) ~1.5GB 1080p versions then started seeding them back up. They did this in a relatively short period starting in ~2019, and you can tell that it was a single person/group because all these encodes of movies include metadata about when they were encoded and the software used to do the encoding; all those movies had the same metadata and encoding timestamps that were pretty close together.
Funny anecdote; apparently the encoding settings they used caused problems for many people, leading to many complaints saying "why do only movies from RARGB stutter?":
I can't offer any hard proof (especially now the site is down) but I've cross-seeded many files to private trackers after removing [rarbg] from the filename and deleting the additional files they add. Looking at my client now, I have two episodes of a TV show ripped by NTb, the TL version of that is just the video file, the rarbg version appends [rarbg] to the filename (to prevent cross-seeding/re-uploading) and includes RARBG.txt and the infamous "do not mirror" file. Both episodes were uploaded to TL ~4 hours before Sonarr grabbed the rarbg release (which usually happens within 20 minutes or so of uploading). If you visit any of the piracy- or torrent-related subreddits you'll see a lot of complaints/comments/concerns regarding "why bother with private trackers when it all ends up on rarbg anyways?" or "why did rarbg steal *upload* from *site* while claiming it as their own?"
