Knit your hat smaller than the head circumference. All knits, ribbing or not, stretch, unless you are doing some particularly stiff stitch as with lots of slip stitches. If the head is 21.5 inches, 19-20 inches is a good size to aim for a finished hat, even a little smaller than that wouldn’t hurt. I’ve knit headbands for my adult head that wrap around the ears, you know, at 18-19" in fair isle (which stretches less), to give you an idea.
Cheesiemom, when you go back and look at the link you provided just above my post, you can see that there are two measurements given. One is the “to fit” head size and the other is “actual measurement”. You will see, then, that for a 21-22" head, the actual finished measurement of the hat is 19.25 inches.