This does sound like one of the crazier leaks, in my opinion -- but a fun one nonetheless, even if I'm skeptical that such a product will reach customers, just like many of the other commenters in this thread.
That said: I think we all know that Apple has a leak problem. This site wouldn't exist otherwise, so clearly a lot of us are quite content for Apple to continue to have such a problem... we like hearing what they're going to announce before they do so. But it should probably go without saying that Apple's top brass doesn't like that at all. As such, it wouldn't surprise me in the least if this is yet another attempt by Apple to ferret out the cause of these leaks. After all, it seems likely that the core of the problem comes from within there own employee base. Apple could potentially fire whoever it is for breach of contract or the like... if they can find them.
In that vein, recent changes could possibly have presented an opportunity. Now that Apple is literally using the same CPU for both the iPad and the Mac, it seems to me it would be mere child's play for a known/trusted Apple engineer to put macOS on a prototype "iPad Pro Max Fold" or some such thing, to show it off to one or more suspected internal leakers.
So, you know... if my crazy theory is correct (which I really think isn't nearly as crazy as the idea of such a device running macOS) it's quite possible that someone either just got fired or is about to be fired.
(Not that we'd necessarily ever end up finding out, of course.)