Early true BIOSen were stored on EPROM, which couldn’t be rewritten while on the board, so those were read-only.
Later BIOSen were often on EEPROM or other chips that could be reflashed while on the board. According to Wikipedia, that started in the mid-1990s. However, you usually needed physical access and/or special software tools to do an overwrite—you couldn’t mount these as a filesystem.
UEFI is quite different from legacy BIOSen and can be mounted as a filesystem, but how much it can be tampered with varies between implementations and devices.
So you would have been correct up until about 30 years ago, but not for modern systems.
Early true BIOSen were stored on EPROM, which couldn’t be rewritten while on the board, so those were read-only.
Later BIOSen were often on EEPROM or other chips that could be reflashed while on the board. According to Wikipedia, that started in the mid-1990s. However, you usually needed physical access and/or special software tools to do an overwrite—you couldn’t mount these as a filesystem.
UEFI is quite different from legacy BIOSen and can be mounted as a filesystem, but how much it can be tampered with varies between implementations and devices.
So you would have been correct up until about 30 years ago, but not for modern systems.