"No information about your text is ever sent to the server, and there’s no way to identify the author based on the minted certificate"

This means the certificate is independent of author and source text.

There's nothing stopping you from sending fake counts/duration to the semoi server. It's a certificate that only says "at this point in time, this is the information I was provided with".

You can then attach it to any piece of text you like.

At the very least, you'd need the ability to prove that there is an underlying event stream with these characteristics, and that this exact event stream creates the document in question. You still can fake that event stream, but it becomes enough work to distract at least casual abusers.

But really, it's the equivalent of saying "I wrote this without AI, honest" in-doc and signing that with your personal key. The value depends entirely on your willingess to be truthful. (IOW: I predict we'll see a resurgence of reputation systems, to some extent)