I saw that briefly while the CD was booting up, before the initial Boot: prompt. What does that mean?

ISOLINUX gives this output when it detects a very bad programmed BIOS.
The BIOS provides basic functions which can be used by boot loaders, DOS, ... When you have a bad programmed BIOS, these functions don't do what they are supposed to do (wrong return value, ...), so ISOLINUX can't rely on them and it has to use workaround (if they exist).

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