It doesn't work like that - if the bees mixed syrup in with stored honey then the water content of the 'honey' would be too high, and fermentation would result. No good for them and nothing usable for us to extract. They don't do this - they ripen the syrup to honey as they would any nectar from an external source. Clearly we could debate whether it is proper honey, since it is from a non-floral source... but they aren't just packing syrup into cells.
The OP is asking about extracted honey subsequently diluted with sugar syrup, IMHO. Different ball game entirely. The Honey Regulations determine such esoteric characteristics as enzyme levels, electrical conductivity, and so on that allow honey to be differentiated from adulterated/diluted honey.