ESXi's mboot.c32 and mboot64.efi have similar names to a multiboot plugin that comes with syslinux/pxelinux, but they aren't meant to be general-purpose multiboot standard bootloaders that could boot a Linux kernel. ESXi started off using the multiboot standard, but we've been slowly making additions (which should be compatible, but aren't tested with anything other than ESXi). So although the newest ESXi mboot should be able to boot old ESXi versions going back quite far (possibly all the way to 3.5, though I haven't personally tested back that far), I would be more surprised if it does boot Linux than if it doesn't.
