To be honest: anyone else could do that. I could put a comment under your article with a link to a Steem post I publish with your text. Do you understand what I mean.. ?

The reference in a Steem post to earlier publications elsewhere also offers no guarantee that you are the author there...

Don't misunderstand: I do believe you, but that won't be enough here.

It's no problem to publish posts that also appear elsewhere. But the authorship must be proven beyond doubt - that seems questionable to me here...