EEPROM. And there has always been a reflash method for bricked firmwares for all AMI and Phoenix BIOS' as well. Insert disk with firmware, Hold a key, power on.
Once upon a time we got somewhere in the neighborhood of 1200 dell workstations and not a single one failed the bios upgrade. 10/10 would do again
I wonder what the practical mixture of NAND flash vs EEPROMs was. I understand NAND wasn't especially stable back then.