I take the comment to mean they don't like stronger flavored fish but prefer milder flavored fish. I don't think degree of freshness is the key here.
I agree with you, Andy. Grouper is a sweet, mild tasting fish, with a texture a bit firmer and denser than cod, but not as dense as halibut or sea bass. Grouper sandwich is a standard on almost any menu in my area that has seafood. Because of its popularity, the catches are controlled, and the price is higher than most similar white fishes.
The most common ways you find it on sandwiches are blackened or deep fried in batter, but you can usually ask for it grilled. The same cooking methods dominate non-sandwich preparations as well.
It doesn't have the oily or fattiness of some of the stronger fish like salmon, bluefish, tuna, etc, so of itself it's not as rich. But since the preparation often includes frying, it can be greasy if poorly prepared.
Several studies (not quoting - some seem legit, some shady) have turned up data that indicates as many as 50% of the restaurants serving 'grouper' are really serving something else. Apparently, once it's battered, fried, put in a bun and loaded with tartar sauce, most people can't distinguish it from haddock or cod, both of which are a lot cheaper than grouper.