It has to do with hardware support for virtualized networking. Instances capable of SR-IOV or with the ENA can sustain millions of PPS. It makes a huge difference.

Not correct. Instances are rate limited based on their type and protocol. Enhanced networking support increases total potential performance, but there are artificial limitations put in place.

They can sustain millions of PPS, but they still get artificially limited, no matter how many ENI's you seem to attach a given instance will top out at around the same amount of PPS

Limits are done on the physical host or the host’s NIC. The TORs are not involved.