My understanding is that followers will only see content from the time they subscribe/follow forward. As Zak mentioned in another comment it's not that they can't, but the platforms choose not to. That said, I don't use the website as my daily fediverse account for a number of reasons. The integrations I use allow for comments, likes, and boosts to be captured, even if they're not replying to a post from my blog account.

Also, if someone looks up my blog on Mastodon they get a message saying previous posts can be found on the original site with a link to my author page.