They waitied until RDNA 2 was feature complete to build their system, Sony did not. There's going to be a disparity in development tools, not only in terms of general availability but also experience with them.
That is nonsense. Direct X 12 is a mature but also very rich abstraction/api.
They would not need to wait for any specific type of hardware to become available before they could work on the dev tools and sdk.
No, any specifics about rdna2 would be hidden beneath DX12 and never be surfaced to the application or the development tools.
But all software is not equal. DX12 is a VERY rich API which provides abstractions for very vastly different hardwares and drivers.
To provide a unified API where you as a dev do not have to worry about specifics about a particular devide or what quirks it has.
This is very nice because you as a developer can focus on other things.
But it comes at a cost. The richer and more layers of abstraction an API provides, the thicker it becomes and the more overhead it consumes, i.e. it gets slower and slower as the API becomes richer and richer.
Maybe that is the issue. DX12 is great since you have the same api both for pc and for xbox which saves development time.
But it is a much thicker and less efficient api than something that only needs to target a very limited set of hardware.
You don;t believe me? Look at a very thin abstraction like Vulkan and how it performs against a very thick abstraction like DX12.
Why is Vulcan so much faster than DX12 on the limited small set of devices that support Vulcan?
Anyway. SDK and tools operate on the API level. The API is exactly the same regardless if you add a new device beneath it.
There is absolutely zero reason why tools would have to wait because one out of many low level device drivers are not yet ready.
TL;DR
DX12 is a great and very powerful standard API. However what makes it standard is also what makes it slow compared to an API dedicated to only a specific small set of hw. Less abstractions to traverse means less overhead.