I'm not a SPDIF expert, so other people can correct me or explain better, but from reading this, it looks to me a bit like they are contradicting themselves...
- they first say voltage above a specific level will give a 1, and voltage below will give a 0
this one is OK, even if we could say "any voltage between the chosen level (threshold) and the highest level"
- they then talk about it like only 0V level was 0/black and 1V only was 1/white, with all between 0V and 1V being grey and not doing anything
There's a threshold, being above gives 1 and bellow gives 0. Even if it takes some time to work, describing it like 0V = 0 and 1V = 1 looks incorrect
So if I'm not wrong, this part is a bad picture of how it works and contradicting what they said just before