I agree, but I don't see how it matters.
So you are saying this temp feed for display purposes is supposed to be able to carry items like electric range, washer/dryer, possibly a water heater (even though there likely is no water connected to the house), other connected appliances, and 3 VA per square foot, even though this is not going to be used as a dwelling during the time it is displayed, and many of the items I just mentioned will not be used during this time either?
If 20 amps will handle all the lighting and nothing else is intended to be used what will it hurt to supply with a 20 amp circuit? Should someone decide to run too much load for some reason the worst thing that happens is you trip the feeder overcurrent protection and learn that it was not set up to run that much load.