See this feature request for ActivityPub API support in Nextcloud Activity App. Related discussion taking place in Hubzilla Developer Forum. This wiki was pasted for reference on possible integrations between both services.
I’d love to see Nextcloud Activity notifications and comments federating with Mastodon and Hubzilla. It would be great to share a file with a Mastodon user, then allow them to send me comments I can receive within Nextcloud. Also see related discussion being mentioned before:
[Testers Needed] Nextcloud Social - Federating with ActivityPub and Diaspora* social networks!