@thosch That’s correct. Posts that start with @ are treated differently by Micro.blog, basically like clicking the reply button. If you want to @-mention someone and keep it as a blog post, you can start it like Hi <a href="https://micro.blog/username">@username</a> instead.
@thosch It’s probably under-documented. @-mentions that start with a username do not get cross-posted like a blog post. I’m checking and it looks like there’s a hole for Mastodon: it does not currently cross-post and @-mention the user unless it’s a reply to a specific Mastodon post. For Bluesky it should work correctly.