I don't know about other vendors but as far as I can tell, Microsoft is the only vendor providing an aarch64 build of OpenJDK for Windows thus making it available to all those cute Windows on ARM machines.
I do not know why microsoft dupplicates this effort.
Why do you say they are duplicating efforts?
The builds on the RedHat page are for: x86 (32-bit), x64 (64-bit x86) but any mention of aarch64, which is what Microsoft has been working on with Azul Systems, is conspicuously missing.
(aarch64 or ARM64 is the 64-bit extension of the ARM architecture.)