Thanks, but no thanks.
For what it’s worth, I personally do not know of any Calvinist who believes that non-Calvinist are not Christian, but I cannot say the converse is true.
I have heard that as the argument goes that if you take the non-Calvinist position, that means that you believe that you contribute to your own salvation, somehow putting your salvation in the *salvation by works* category.
IOW, the only alternative to Calvinism is salvation by works ergo, if you are not Calvinist, then by default you believe in salvation by works.