intel management engine and other hardware backdoors were never exploited by any actor of any caliber successfully enough to make any difference — the weakest link was always something else. the biggest negative impact built-in hardware backdoors have on privacy as an institution is that when people find out they exist, they say "okay, they got me, there is nowhere to run, so I can either go live in the forest or accept my fate and carry on using windows."
that was exactly their intent all along. in reality, you _do_ have a choice. using Linux _does_ save you from their eyes.
hardware backdoors were _never_ successfully exploited.