That’s not true. What you’re disabling is the interface by which the main CPU can talk to the PSP. The thing itself is still very much active.
Unfortunately, and I say this as someone who’s used only AMD processors since 2004, Intel makes the most trustworthy modern x86 CPUs. This is purely because of the ME Cleaner software which allows you to gut the ME and rip out everything except the most basic initialisation code (there’s also the so-called “HAP bit”, but I don’t fully trust that it actually does disable the bastard).
If you’re OK with looking back a couple of generations, the Bulldozer core CPUs (AMD, socket AM3) are the fastest x86 chip you can get which still allows the user to maintain full control over it. I’ve recently upgraded my two main machines to Ryzen chips, and I’m explicitly holding on to my old FX-8350 builds for exactly this reason. I know that they have a separate CPU inside them as well - the System Management Unit; and that the SMU’s firmware is cryptographically signed… but it’s a symmetric crypto signature and there was an exploit in an older version of the firmware which allowed you to get this key (abstract and video).