In this case, the problem is that you're referring to an old post that has never existed on the Micro.blog timeline, so there's no entry there for the plug-in to link to. And no, you can't really comment on a blog post that has no Micro.blog timeline entry. You could send a webmention.
You can change the fallback text to something that makes more sense to you, like "Write me on Micro.blog", or disable it completely by unchecking Show fallback link in the plug-in settings.
Thanks for pinging me, @SimonWoods, I might have missed this one otherwise.