Activitypub goes in both directions. For example, people using Mastodon will be able to follow tumblr users, interact with their posts, comment, share it on their profile, all from Mastodon apps. And vice versa. Same applies to all services that fully support activitypub. Another example - right now you can follow a peertube channel on mastodon and see their videos in your timeline as if they are using mastodon, not peertube. If you post a reply on mastodon, it is automatically shown as a comment under the video on peertube.