Octatonic scales don't fit into keys - I think the section talks more about how there are only three total variants of the octatonic scale. In other words, no matter where you start playing from, you'll be playing in one of three unique sets of notes. It is true though that the scales "C, C#, D#, E, F#, G, A, Bb" and "C#, D, E, F, G, Ab, Bb, B" are the same interval pattern, just transposed one half-step. So there's only two possible interval patterns (half, whole, etc. or whole, half, etc.), but three distinct groups of notes that you could be playing in (starting anywhere, not just starting from C).