boot.wim is generally located in the sources directory of the image. The "echo" showed the correct path, but the initrd command attempted to load from the wrong path.
@@ -48,7 +48,7 @@ initrd -n bootmgr ${win_base_url}/${win_arch}/bootmgr bootmgr ||
initrd -n bootmgr.efi ${win_base_url}/${win_arch}/bootmgr.efi bootmgr.efi ||
initrd -n bcd ${win_base_url}/${win_arch}/boot/bcd bcd
initrd -n boot.sdi ${win_base_url}/${win_arch}/boot/boot.sdi boot.sdi
-initrd -n boot.wim ${win_base_url}/${win_arch}/boot/boot.wim boot.wim
+initrd -n boot.wim ${win_base_url}/${win_arch}/sources/boot.wim boot.wim
boot
:windows_exit
driz