Yes, that would be nice, although I’m not sure how implementation would work. You need something to anchor the toots to the post. The logical choice is the URL of the post. However, this can be accomplished already with webmention and doesn’t need ActivityPub at all. You have to use something like Bridgy to monitor your account for your domain and then pass them to webmention. Unfortunately, Bridgy can be a little finicky.

I used to use a WordPress plugin that allowed people to post comments using their social media accounts, but that was just for verification and it wasn’t an integration with social media.