AWS was not available to developers within Amazon to use at any time before the day it was publicly launched. At least not on a broad basis, and none of the main Amazon.com services ran on it in 2007.
They did share some data centers, however. And I guess data centers are "infrastructure".
I was in a position to know this because I had my hands deep in the retail site as part of my job.
I'm not saying that nobody in Amazon wanted to use AWS, just that we didn't know about it until the press release, and so there wasn't any opportunity to use it... and of course, all the code from the 1990s was built in a different way so it would have been nontrivial to migrate it.
tl;dr The organic migration to AWS started in 2006 and continues to this day.
http://psav.mediasite.com/mediasite/Viewer/?peid=7ab95f6a5d4...