As far as I can tell, there's no concrete evidence that it is actually an intentional "backdoor."

What would you require to feel confident it is a backdoor?

Nadella gives a press release, "Alright guys, you got us fair and square. Backdoor on Bootlocker. Various versions of it for years on behalf of the spooks."

You are unlikely to ever get a confirmation of wrong doing. That being said, for a first line security posture, there is no way external media should have anything to do with the encryption process. Even if the OS chose to read a USB drive, to also delete the magical files is ridiculously suspect.

It could always be plain old incompetence, but that is a damning level of technical ineptitude assigned to such critical infrastructure. This is not a project you assign to the intern, but paranoid security experts. Multiple levels of code review and red-teaming.

lol it’s an obvious backdoor. No way a security system would ever allow this blatant workaround to bypass all encryption. Backdoor is the only answer