It could only be an imperative as "Thank me/him/her/us/them very much". A speaker can't demand that "you" thank "you". If, in a very convoluted context, you need to tell someone to do that, you'd say "Thank yourself very much!"
Yes, absolutely. I don't know what led me to say that it arguably is. There's no argument to be made at all. That was a case of impatiently responding without thinking carefully about what I said.
Sorry, sitifan, to dismiss your question like that. It's a perfectly reasonable one.