I originally also subscribed to the theory that the thing added to lesser to make a platy was recessive but with more breeding results posted that doesn't seem to be the case.
1. RDR made the butter version of platy without inbreeding. So it must be just one additional allele to turn a lesser (or butter) into the platy.
2. No lesser X lesser has been reported to produce platy. If the extra ingredient was recessive then surely since all RDR line lessers are descendent from the platy a pair of hets would be bred by now. The allele theory explains why no lesser can carry the extra gene to make platy.