Citation
They are correct on the independent network access, but the 3G part is tinfoil hattery:
https://software.intel.com/en-us/articles/intel-active-management-technology-start-here-guide-intel-amt-9
The ME does have its own MAC so the paranoid can block it at the firewall.
https://software.intel.com/en-us/articles/intel-active-management-technology-start-here-guide-intel-amt-9
Notice that there is a network connection directly associated with the Intel ME. The specific NIC will change according to which Intel AMT release you are using.
The ME does have its own MAC so the paranoid can block it at the firewall.