I've not got your board, so can't see the screen you're seeing. The X11SPH0.B03 file inside the ZIP file should be your BIOS file though. Even though it doesn't have a .BIN extension, it is a BINary file type, so your system may (should?) load it for update just fine.I did see firmware updates in the bios menus so downloaded the latest update. The firmware update ipmi page had a place to upload a file. It said I couldn't upload a .zip, only a .bin. Fine, so I unzipped the download but there wasn't a .bin.
If you've got a monitor attached for that initial install of Proxmox, you could also update the BIOS (and also IPMI I believe) the "old school" way. In this case though, instead of booting from a "DOS bootable" USB (or Disk if you're really old like me), you need to boot into the motherboard's built in UEFI shell, and then run the BIOS update from files saved to a USB flash drive. Take a look at the instructions in that Read Me file in the section "Standard BIOS Update Procedure under UEFI Shell" .
Cheers!