Amazon has good developers, but the JVM is quite complex and the pool of people who can work on it is pretty rare, I wonder how well they’ll be able to maintain it on their own.
At this point it appears to largely be a vote of support for openjdk as the project evolves.
From their announcement:
Amazon has already made several contributions to OpenJDK 8 and we look forward to working closely with the OpenJDK community on future enhancements to OpenJDK 8 and 11. We downstream fixes made in OpenJDK, add enhancements based on our own experience and needs, and then produce Corretto builds. In case any upstreaming efforts for such patches is not successful, delayed, or not appropriate for OpenJDK project, we will provide them to our customers for as long as they add value. If an issue is solved a different way in OpenJDK, we will move to that solution as soon as it is safe to do so.
Several old (e.g. SUN) Java devs have left Oracle in the last years, so there's that.