The term has little meaning. You could call socks "military-grade" as well, and the only meaning that it really conveys is that it was cheap and barely functional enough to win a contract.
And AR15s are cheap. A lot of plastic and aluminum. You could say that's a problem, if you want. But that line of argument doesn't seem to lead anywhere.
In any case, "military-grade" seems like a talking point and nothing more. If anything, it gives the rifle more protection under the second amendment, which specifically references the militia.