APIs are hard to get right the first time. I could see why they wouldn't want to release one until they've dogfooded and refined it.
That said, I'd love to see them take an approach unstable API release that requires the app to show a warning like "This app relies on unfinished features that may change or stop working entirely in the future, requiring the seller to release an apo update." and require them to launch it as a free preview, make it refundable during this period, etc.
I feel like we could expect a bit more from the billion dollar company to support new apis. They have a solid mechanism to deprecate apis and force developers to rebuild their apps already, just shorten that window for the cutting edge apis released in preview.