That is not true at all. A NEMA 14-30 receptacle can absolutely be used with a 30 amp breaker. In fact if you put a 40 amp breaker on a 30 amp receptacle, you would risk starting a fire because your receptacle is only rated at 30 amps.
What needs to be limited to 80% of the circuit's current is any continuous load (i.e. an EVSE). He can only plug a 24 amp EVSE into this outlet. Like a Clipper Creek LCS-30P with a NEMA L6-30 plug to match the receptacle he has chosen.
The biggest mistake the guy made was not using an old work box to protect the receptacle in the wall. Several other things people have pointed out are valid as well, but he did choose the correct wire, breaker, and receptacle.