Clearly when Amazon realised the enormous potential in AWS, they scrapped that principle. But the idea behind it - that an organisation used to fat margins will not be able to adapt in the face of a competitor built from the ground to live of razor thing margins - still applies.
AWS is ripe for the picking. They "can't" drop prices much, because their big competitors have similar margins, and a price war with them would devastate the earnings of all of them no matter how much extra market share they were to win.
The challenge is the enormous mindshare they have, and how many people are emotionally invested even in believing AWS is actually cost effective.