I'm currently unable to tell which system ProxMox is using to boot. I know that the BIOS boot type being UEFI or legacy should be an indicator, but I wonder if there is a way to verify this on a working system.
Looks like I was indeed using systemd-boot, thank you for explaining. This was interesting to me because I have the BIOS mode set to legacy boot (HP z420) which I thought would force a grub boot. However since I installed with ZFS as root it must have been able to use UEFI anyway. As a followup, what does the "boot to firmware" option in systemd-boot do?