It requires understanding that Mastodon is a product built on a protocol. The question is do you want your content on your blog to be something people can subscribe to using that protocol, just like someone can use RSS to follow your blog versus visit it, or do you want your content on your blog to be copied to the Mastodon product on your behalf?
The difference is crossposting, which works just like cross posting always has and subscribing, which is like RSS.
The underlying protocol that Mastodon (and Pixelfed and Lemmy and whatever other “fediverse” products) build on has a mechanism for subscribing which is meant to be more real time than RSS. Having ActivityPub enabled means people can subscribe to your blog just like they can subscribe to RSS. The main difference between RSS and ActivityPub (other than push v. pull) is that ActivityPub also specifies a means to reply to messages you subscribe to. Micro.blog could have ignored this and just treated ActivityPub as a mechanism for subscriptions only, but it went further and will respect those replies and make them appear like Micro.blog replies so that you can view and interact with them.
RSS has no reply to mechanism. The collection of protocols sometimes called the Indieweb does have an answer to the ActivityPub reply which is both simpler and less robust in some ways, known as WebMentions. This is a separate protocol that Micro.blog has also supported for some time. Webmentions are not as widely adopted as ActivityPub at this point, and are generally not coupled with RSS.
However, it’s entirely possible to build an RSS reader with a reply function that results in posting a reply on your own blog (or someone else’s web service) that sends a webmention that may or may not be handled by the original site. Sites like Bridgy essentially act as a way to convert other forms of replies and mentions into webmentions.
I think a lot of folks on Micro.blog (and off of it) are deeply confused about ActivityPub the protocol, and Mastodon the service. But really, it’s not that different than Micro.blog before versus Twitter crossposting. If you crossposted to Twitter, you just wrote a native tweet that had nothing to do with your original blog post once sent. It was just Micro.blog being a computer in the background who took your post and turned it into a tweet that lives on Twitter and can be replied to on Twitter. That’s what crossposting to your existing Mastodon account does.
Mastodon, however, because it uses ActivityPub, is able to interact with products that are not Mastodon more natively. ActivityPub support means people in Mastodon can see your actual blog post on Mastodon and can reply to it on Mastodon while your experience remains completely contained within Micro.blog’s ActivityPub handling.
I would love to know what is confusing about this support in general, because I think the language that exists is broadly clear, and more than that, the experience “just works” such that very little of this seems to matter, practically. But I’m sure Manton would love to hear what words people are looking for at what stage to help people make a decision (though I don’t really think there are many downsides to just using Micro.blog and not really thinking about it past that).