The flatness of the stone, does not matter that much, because we are not honing on the exact same point with each stroke, that it is smooth, is more important.

A flatter stone is a bit more efficient, as more grit is in contact with the razor, but nothing that an extra stroke or two would not compensate for.

The age-old question, how did the old guys get their stones flat, before dial indicators and diamond plates? And how flat is a grinding wheel or a spinning sanding belt?