It's only partially true though. There are tons of teams that Amazon that do not use AWS. They use legacy internal data centers that do not have an ounce of AWS infrastructure. And for some reason it is nearly impossible for them to transition.
There are a ton of these teams. AFT (several hundred devs), for example, is practically on their 10th year of a 3 year mandate to get off of their monolithic Oracle database backends, a constraint which forces them to run in legacy datacenters. You'll find similar issues with plenty of Supply Chain, Operations, and Transportation teams as well. The primary data warehouse clusters (non-redshift) and management interfaces are also in legacy data centers, last I recall.