The mandate was in 2001. AWS first launched in 2006. From reading the letter, it wasn’t about being able to make services productizable. It was more so teams could work independently and choose whichever underlying technology they wanted. It also prevents the issue the original poster was having. It’s much easier to make changes to a small API than a monolith.
It takes a lot to go from internal API to product even on a small scale. Werner Vogels said at the last Reinvent that S3 is made you of hundred or more separate internal services exposed internally via an API. My last company we also had a mandate to be “API first”. Not because we were trying to be like Amazon with less than 75 people, but we actually sold access to our APIs to our customers - large business that used them as the backend for their websites and mobile apps. We also used the APIs internally for our websites and large ETL jobs where sour clients would send us files for bulk changes.
This was Bezos' reasoning for making as much of their internal software available to the public via AWS
Those APIs had nothing to do with AWS. So how was the original poster not talking about AWS when they mentioned AWS?
Also a clean public interface says nothing about how badly written and documented the underlying code is.