@jbowen @lanodan This piqued my curiosity, so I looked into the history of the project and found that it was relicensed from Apache 2 to AGPL3 last year (https://github.com/matrix-org/dendrite/discussions/3281). So the explanation that seems most likely to me is that whoever updated the license just forgot that it was mentioned in CONTRIBUTING.md. I don't think one can reasonably assume it's intentional, at least not based on what I saw.
If somebody submitted a bug report pointing this out, or a PR fixing it, and that wasn't well received, then that's a different story - it would start to look extremely suspicious.
I do have to wonder what the terms of the commercial licenses they sell are... if the software isn't delivered to commercial customers under AGPL3, that "smells" an awful lot like copyright infringement. (though I'm no lawyer)