Or is the real story how Windows 11 was rushed to market barely finished and they're just going to slowly add features over the next few years?

This. The new Media Player is actually a complete rewrite, and older installs of Windows 11 used the old WMP 12 instead (which you can still install).

I just think it speaks to the entire mess that is Win 11. They seem dead set on replacing things before the replacement is actually finished.