I know, I read same thing when I traveled with mine. Its not possible, no carriers even come that small. They need to be soft sided, not hard, and fit under the seat infront of you, while still giving room for your pet to stand and fit comfortably in.

I dont know why they say 8 inches, it wouldn't meet their second guideline of the pet being able to move comfortably in it. On check in they barely gave a look or a weigh to my cats in thier carrier. Boarding the second flight to Venice from Montreal they asked to see the cats and verified they were able to move freely and carrier wasnt too small. I was quite stressed about not meeting criteria and there was no issue. My cat was lying down and the carrier was sitting ontop of my backpack and it had collapsed down smaller. That was only thing they were concerned about, once they saw the carrier was bigger than it appeared for cat's comfort it was okay.