It's part of the Social Group at W3C IIRC. WebSub solves problems around RSS, notably that clients have to poll a lot and smaller sites thusly need to maintain those requests.

WebSub solves that by designating a hub that can handle this instead. Ie, you federate your blog feed to elsewhere.

ActivityPub fills another niche, mostly everything around human interaction in social networks.

WebSub could be used to feed data into ActivityPub networks.

After reading the responses I don't see yet why the non-transport bits of WebSub can't be done with ActivityStreams and ActivityPub. That said, I haven't put the kind of time into it like the W3C Social Group has, nor am I recognized expert on it, so I'll hold further comment until I can finish and publish to Show HN a related proof-of-concept involving Peer-to-Peer topic based publish/subscribe using ActivityPub, that I've had on the back-burner for a while.

See it more as an alternative to RSS. Aggregators subscribe and feed information to their users. The polling step is just switched to pushing.