It's an open-source comment system federated on the Matrix (https://matrix.org/) network. This means you can use your Matrix identity to comment on any site that uses Cactus, without the tracking of something like disqus. Works well with static site generators too.
In particular, any federated identity provider has a problem for this use case in that malicious actors can simply create their own domain - or multiple - and spam/troll from those. Blacklists essentially don't work as long as new domains can be created, so you end up with a whitelist, which kinda counteracts the federation concept.
It needs something where getting blemishes on your ID is actually something you want to avoid. And where fresh IDs is not effective to bypass this.
There aren't too many useful or any toxic comments, but I attribute that more to the lack of readership and interest than anything else.
OAuth and (shudder) "OpenID Connect" moved on to be nothing like that original OpenID and its vision. I think the death of the original OpenID is tied in part to the death of comment areas on blogs. It wasn't the only reason, but it was a factor in tide of them.
This is solved either by moving commenting entirely off-site to a larger network, or by using identities from a larger scope.