Contrary to the others, I would read it to mean mayonnaise, not Miracle Whip.
Some would not think of mayo as salad dressing, others would.
Some would equivocate mayo and MW, others would not.
So I am thinking it was just this person's way of clarification.
Plus ... today, most would never think of mayo other than from the store.
Mayonnaise was commonly made at home, in days gone by!