I was under the impression that before AWS, Amazon did SOA'd almost the entirety of its departments, as in OP mail, every dept had to enable web services to other depts, creating an environment where

Between 2000/2003 Amazon started to learn how to properly make something that resembled AWS (launched in 2006), it was a necessity targeted towards scalability. What Yegge letter meant is that Amazon was doing SOA to all its depts, making these be useful both internally and externally

>"5) All service interfaces, without exception, must be designed from the ground up to be externalizable. That is to say, the team must plan and design to be able to expose the interface to developers in the outside world."

So you see, it wasn't to enable AWS as the digital service as we know it, it was to build a scalable company that could use everything it had both to serve its needs, and also to sell it as a product (then, all services we all know EC2 etc). Bezos applied SOA as a company, not only to its digital philosophy. And this corporate move, with no precedent, enabled not only AWS, but the whole Amazon as it is today.