What does the enable/disable ME subsystem option in Supermicro X9DRi-F motherboard BIOS do?
"It's well known fact that the Intel Management Engine can not be completely disabled" - as far as I know, it can be disabled by manufacturers at least. See this link about Dell's purchase option: systems with ME disabled could be bought from them: https://www.extremetech.com/computing/260219-dell-sells-pcs-without-intel-management-engine-tradeoffs
So what does this bios option do? It disables ME. There are obviously systems on the market that allow it. Try it out, disable it and see if ME vanishes from device manager.