The original statement about BIOS -- that it was a second OS from the first day on -- is not correct.
I am pretty sure the term BIOS (Basic Input Output Subsystem) comes from the early days, from CP/M. That's when I started hearing it anyway.
The BIOS was the bottom half of CP/M. It provided an abstract device interface that the top half -- the part that came on a floppy -- could use. So the BIOS was not a second OS. It was 1/2 the OS you ran. Vendors created hardware and BIOS for the (usually 1024 byte) ROM and that ensured that CP/M could run on their box. The part of CP/M on the floppy, and the part in EEPROM, combined to provide a kernel. Kernel, of course, being a very loose term given the lack of processor modes.
This was a very different model from, e.g., the minicomputers of the day, wherein the ROM was used to boot a kernel and then it got out of the way.
So BIOS nature as a runtime entity was established very early on.
Linux, for a long time, did not need no steenkin' BIOS (you can find that quote if you look back far enough). But those days are gone.
ron