I think if someone on Mastodon comments on a Ghost blog, then Mastodon will tell Ghost that they did so (via ActivityPub). Then if someone on Pixelfed sees tha

I think if someone on Mastodon comments on a Ghost blog, then Mastodon will tell Ghost that they did so (via ActivityPub). Then if someone on Pixelfed sees that blog, Pixelfed will ask Ghost who has commented on it (via ActivityPub). Ghost can then tell them about that (via ActivityPub) - or it can choose not to do so (based on its own moderation tools). That doesn't mean that the Mastodon comment no longer exists, nor that it is invisible to the people looking at the Mastodon comment (similar to HN comments), but Ghost won't boost its reach. So what ActivityPub gives you is the option to reshare comments.