Because parts of it have already been reverse engineered, we know it runs a modified version of Minix, and I would think that if a backdoor had been found during the reverse engineering process, that it would have been huge fucking computer security news.
It's only a backdoor in the sense that Intel was practicing security through obscurity instead of real security. There is proof an attacker could abuse the IME, but there is not proof it's an intended backdoor for use by Intel in spying on their customers.
EDIT: Further, as an all-AMD user, I almost never see this same scrutiny applied to the AMD Platform Security Processor. We know far less about it, and it deserves the same level of scrutiny, honestly.
Here is an alternative Piped link(s):
https://piped.video/watch?v=LzWy49LeI3U
Piped is a privacy-respecting open-source alternative frontend to YouTube.
I'm open-source; check me out at GitHub.