I think thereg didn't get that quite right. From reading the original advisory, I would conclude that this is not a backdoor (a by-design security "override"),

I think thereg didn't get that quite right. From reading the original advisory, I would conclude that this is not a backdoor (a by-design security "override"), but an actual vulnerability in the boot loader, where it does not check the type of the signed data blob (policy vs. supplemental policy) and thus can be exploited to disable further signature checks. The effect is the same of course, both to attackers and to device owners. One could speculate if this is a backdoor created with plausible deniability, maybe paid for by some three letter agency. But the evidence doesn't really point into the direction of an intentional backdoor.