I hope I'm not missing something, but here's the message when I try to access the thread.
I wouldn't be surprised if this signature is disallowed in line with the mixer promotion ban. Mixers are certainly an important part of the site, so in a way it can be considered as a promotion of links to mixer sites.It looks like a match to this
But there seems to be some confusion around promotion. It would be allowed to post even a positive review thread for ExampleMixer, just as long as you don't post the UR]L and you don't violate the other rules. Promotion is only disallowed in signatures, avatars, and profile-bios; not posts.