Agreed, Food is not useless. It's just a rule of thumb to make it easier to remember on the priority scale. Maybe overstated a bit because Civ 6 is the first Civ game where more people wasn't an absolute positive. Decades of past history has led to an ingrained thinking to "grow, grow, grow", and having a contrary rule of thumb helps to check that automatic response and make you think through the specifics of your current situation.
It will be interesting to see how game play could be different with the Inca given their huge boost to potential food production but zero boost to amenities. As
@pgm123 pointed out, amenities are the ultimate check on how big a city can be. As you say, higher yielding tiles may allow the Incas to generate their Food with fewer Population in the field, letting them allocate more Specialists even if their overall city population is the same. Which is why my initial comments on the Inca were that if GS boosts the value of Specialists (and I hope it does), then the Inca could be very powerful.