9 times outta 10, when you get somebody vagueposting about "I was fired for NO REASON," they're just incapable of actually admitting the reason.
I am hesitant to share company DMs for obvious reasons.
The advice to seek legal representation is still probably your best course of action. If you're clearly in the right and have a paper trail showing it, you have a chance. Of course, the first thing your lawyer will advise you to do is take down the Medium post.