The exception that Apple notes in its list is 'not all ...'. As examples of possibilities:
Built-in video management can be affected by PRAM setting if 1-bit depth (B&W) can be realized by the CRT, and if that parameter is stored in Parameter RAM.
If a Mac uses a video card—even someone else's proprietary chip in a 'pseudo-slot' on the MLB—instead of inbuilt video, the card has its own remembrance and instructions about bit-depth, and does not consult PRAM at startup.
If the CRT or video circuitry is not capable of 1-bit depth, it cannot be instructed to use it.
I don't know the 630CD, but if you look up its specs at Apple, you may find the answer to your puzzle.
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