To be clear, this is only for the ability to update the BIOS over the BMC interface, not for BIOS updates in general. (unlike some other vendors, where you need a support contract to be even able to download the updates)
At least on some boards you can boot the USB drive image containing the BIOS updater through the BMC and do a remote update that way.
Supermicro doesn't publish changelogs for their BIOSes. Their disclaimer on their BIOS download page says, "don't update the BIOS unless you have to". It's a ridiculous stance for a server company. You buy Supermicro over Dell for the price. That's pretty much it.
For the models I was looking at a number of years ago, the options were through IPMI or via a USB Floppy Drive. Perhaps my memory is failing me here, but I seem to remember being quite enraged at the prospect and would have done a lot not to have to pay the license fee on principle.
At least on some boards you can boot the USB drive image containing the BIOS updater through the BMC and do a remote update that way.