I'd guess that if anything they'd increase the pressure to raise the boiling point. That way things dissolve in it faster and the water doesn't evaporate away.
So the old grading system rated the lighter colored plainer sugars and syrups higher than the darker more flavourful varieties. Since less maple flavour made for a better all-purpose sweetener and a more direct competitor to cane sugar.
Nowadays we usually use maple syrup for the flavour, so the grading system is non-judgemental that way. And the darker grades are more likely what you want.