This AV1 video encoding layered upon DX12+HMFT for cross-vendor support on Windows is done to benefit Microsoft's continued ambitions around Windows Subsystem for Linux (WSL) support.
The code merged today for Mesa 26.2 provides a "prototype" implementation good enough for I and P frame encoding but not any advanced features until further integration takes place. This new Mesa code was worked on directly by Microsoft engineers.
Getting this AV1 encode prototype up and running was just around 900 new lines of code on the Mesa side. Those interested can see this merge request for the code now upstreamed for this quarter's Mesa 26.2 release.