I think this is one of those cases where you need to take into account the intent of the Intel ME and whether or not you can consider it a backdoor. Surely it's a useful tool in corporate environments but to any other average individual it's definitely a backdoor. It's a "feature" of nearly every modern x86 CPU that undoubtedly has capabilities of a backdoor that cannot be turned off or disabled by regular means. If I wanted to be able to remotely manage my machines out of band then I would've asked for it, but instead I foolishly bought myself into a very easy way for vendors to maintain control over me and my data.