I can't imagine anyone caring enough to make a connection to a stranger to bad mouth someone, nor can I imagine someone remembering an interview from years ago about a candidate they didn't hire. Also if I understood correctly, the interview with the guy was about 3 years ago, that's about when the interviewer connected with the owner of the IoT company on linked in, and when you were having the conversation with that owner yes? So this is all a hypothetical about a thing that may have happened three years ago? Why are you worrying about this now? Just let it go and don't hold on to the past, you can't change it. Just write both of them and their companies off of possible interactions.

That said, let's assume that this interviewer is actually going around bad mouthing you. Why don't you hit him up on LinkedIn? Preface it with "You may not remember me, but..." and just tell him that he was right. Say that his feedback while mean, helped you realize you needed to grow as a person and developer. Make him understand that you are sorry for how you reacted in the interview many years ago. Make it clear you're not asking him for a job or anything though, and that you just want to thank him for his helpful feedback.

It will stroke his ego a bit, and in my experience when you admit you were wrong even when its been years, and show how you've rectified the problem, the person is willing to try and reform an opinion on you.

Though I would bet on the first option of the possibilities you stated.