> an arbitrary rule that you just made up.

No, it's an observation that the first primary usage seemed to disagree (not that it did) and so it was observed that the second alt was used by the commenter above

OED has a lot to say about gourmand, Chesterfield in his 1758 letter that you quoted was saying that the Landgrave has a well stocked table .. good food and a lot of it, for he is a Gourmand. Following that Chesterfield example is a 1816 Coleridge extract from Statesman's Man that also about having a lot (but with no talent for preparation) - excess over taste:

  Their best cooks have no more idea of dressing a turtle than the gourmands themselves

And, again, the first 1a primary most common usage cited in the OED is:

  1. a. One who is over-fond of eating, one who eats greedily or to excess, a glutton. 

It's a usage that has morphed in recent times, sure .. but as seen in the OED for a great deal of time the emphasis has always been on the quantity of good food rather than mere quality of good food.