Intel probably had to disclose the existence of IME due to collaboration with mainboard vendors. Apple does not face this constraint, so it is a lot easier for them to keep such subsystems under wraps.
Of course I'm just speculating here, but a product typically mirrors its environment.
I find it implausible that the A/M series chips have an independent subsystem that is so obfuscated that the expert attention which each Apple die receives has turned up no trace of it.
The company has its own approach to secure compute with the T2 modules, but no, I don't believe Apple would be able to hide something like IME on their CPUs without it being detected as such.
https://en.wikipedia.org/wiki/Intel_Management_Engine
And this for the T2:
https://en.wikipedia.org/wiki/Apple_T2
Neither of these are obscure products, they are of great interest to reversers and other security researchers. The list of shady things IME does which the T2 isn't known to is extensive.
The number of expert and curious people, with the means, is higher than the number of new chip types Apple or Intel produces. There's always a detailed die photo available within the first few weeks of a product launching.