A supervisor can easily not have "enough clout in the field to deter blatant research theft". If you're a tenured professor from a random university in a random country and are not among the top few names in your field, you probably can and will supervise theses, but good luck convincing an editor from an elite institution to care about you.

And in your last two paragraphs, I think you're being a bit naive... what if the editor in chief is directly involved, or is a friend of the plagiarizer? Whom do you turn to? How do you prove that you did the experiment first if there is no proof but the submission, which is in control of the editor in chief?

I fortunately have never been the victim of such a despicable ploy, but I have seen all sorts of malpractice in journals, including editors that didn't care about blatant plagiarism. I don't think it's a predominant thing, fortunately there are plenty of honest journals and honest editors. But it happens. I'm quite familiar with scientific publishing, having authored a good number of papers, and I find the story perfectly believable.