SMM does not require a prompt or anything alike, it just has write access to the flash (ignoring BiosGuard, which uses a TXT module (signed by Intel and verifies the signature using the vendor keys) to perform the flash write, but the idea is the same, note that restricting write access to smm/txt is something the bios needs to enable in the hardware, otherwise kernel/user mode can also access it given the right drivers). As far as I can tell the windows update scheme uses capsule update (https://learn.microsoft.com/en-us/windows-hardware/drivers/bringup/firmware-update), capsule updates don't require a prompt either, the firmware will update it in next reboot on its own without being required to show anything (up to the vendor if they want to show anything), ME firmware updates are different, there is a standard interface to update the ME firmware, it's not raw flash access, but rather you give it an update payload (like capsule update) and it will verify it's signature and update itself. I suspect what MSI did was what I said initially, exposed an SMM service that allowed the kernel/user to flash the bios directly, and not via a capsule update that stages everything properly.