That statement sounds strange to me.
Normally 11 kV / 400 supplies, if metered at the HV side, are metered in the 2 wattmeter configuration which generally meters the Red and Blue phases on the 3 phase delta side of the TX.
According to Blondells metering theoreem any poly phase circuit will be accurately measured with n-1 metering element.
Where n are the amount of phases, in this case 3 minus 1 equals 2 metering elements.
The VT is normally 3 phase and provides 110 Volts for the meter, of which the yellow phase is earthed for safety.
th CT's are often 5 Ampères and sometimes 1 Ampère in case of long wire looms between and HV supply and metering point.
I think that the person is confused with the occasional cowshed, or irrigation pump supply with a polyphase motor on 400 Volts.
These often get metered with one single phase meter direct or a single phase CT meter and CT on supplies over 100 Ampères.
