Then again, the relationship between AWS and Oracle is even more contentious and Aurora MySQL is one of AWS's most popular products so I don't think they are terribly worried about building on competitor's technologies.
At least when I was there, the strong focus was always on adding new features (global & local secondary indexes, change streams, cross-region replication, and so on) to keep up with the Joneses (MongoDB et al).
Meanwhile, a bunch of internal Amazon teams were taking a dependency on it instead of being their own DBAs, and those teams didn't care that much about the whiz-bang features, they just wanted a reliable scale-out datastore that someone else would get paged about when some component failed.
Adding features at a breakneck pace while keeping up umpteen-nines reliability and handful-of-milliseconds performance meant tech debt and non-user-facing improvements, including WiredTiger, all got sidelined. Around the time I left, our page load was around 200 per week. That's one page every 50 minutes, 24/7, if you're keeping score at home.
I would love to get a behind the scenes look at the process of gradually improving the components of DynamoDB with better technologies, while still maintaining reliability and performance.