After playing with it quite a bit, it seems that—if you reply to a specific micro.blog post using a link (in this case: https://micro.blog/jd/820446), it’ll add the @-mention to the beginning if you send a Webmention. And you can also force the @-mention(s). So, if you’re mentioning several people, you’ll want to put those on—and, in that case, it’ll leave them however you have them.
Great work on your site! I also use Jekyll and have been working on a fork of the ‘jekyll-webmention_io’ plugin that adds a bunch of features. I wish Jekyll was faster—my site takes over a minute to build now. Maybe I’ve done something. I have.