It means no pixel information in the white. You'll have to excuse me, I worked a lot in repro and often retouched in CMYK not RGB, when there is 0% in Cyan,Magenta,Yellow and Key, it was said to be zero-dot, i.e. no pixel information and therefore completely page-white. Desaturate only means to pull colour out the image but it can still have tone, effectively its a black and white. I feel I may be confusing the issue. Just make sure that you have a good tonal contrast range from black to pure white.
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