While they seem different, they can be used in similar ways. Sorta like how Delta Chat turns email into something like IMs with a conversation view. It's all passing around text/media at the end of the day.
Yes, quite different concepts, but both are federated services that offer the necessary functionalities to be used as comments. One is a bit more "real-time" than the other.
I should perhaps also add a little disclaimer/warning: When i had a closer look at cactus comments a while ago, it seemed to be broken / "assembly required" in various parts and blogs that use it.