I'm just jumping into this now but you're not really listening to what the parent is saying. You seem to be acknowledging their point but then ignoring what they're saying.
No one is talking about what should happen. They're saying that, as you've confirmed, m.* is a separate resource from en.* because they exist as 2 separate pages on wikipedia.org. On the en.* page, people on mobile are incorrectly redirected to m.* which it seems that everyone agrees shouldn't be the behavior. The opposite, m.* redirecting people on desktop, doesn't occur.
That means that your statement that "URLs should be for a specific resource" contradicts your other statement that "sharing that URL ought [not] to send anyone to the mobile design". If the mobile page is a separate resource, then sending someone a link to m.* should always load that resource (the mobile version) which seems to the be the opposite of what you're suggesting.
Additionally, there seems to be additional confusion because everyone is assuming that the page loaded at m.* is the same page as that loaded on en.* but that doesn't seem to be the case as the page isn't responsive. It's the same text content but the stylesheet and headers are different which, by web standards, is a separate resource.