At a minimum, it's a thing that certain security conscious consumers (cough DoD) were able to get Intel to include a hidden (not typically user accessible) bios flag for disabling most features of the management engine. So they're at least concerned about it as a security risk. That doesn't necessarily mean they also have backdoors into it, but it's not crazy to think they might. It's hard to be too conspiratorially minded with respect to intelligence stuff, if you aren't making the mistake of treating suppositions as facts.