The page in the link actually says 'With 256 notes of true stereo polyphony, available notes will never be cut in half just because stereo Voices are in use, and even dense arrangements and performances won't brush the ceiling.'
All that this means is that a voice can be mono or stereo but will effectively use the same polyphony.
However, although the statement in the link does not admit it, after pressure from users Yamaha have admitted that the 256 notes of polyphony are actually split between 128 (max.) notes for internal voices (including USER edited ones, stereo or mono), and the remaining 128 notes for expansion voices! The current OS does not make the extra 128 notes of polyphony available to internal voices when no expansion voices are being used, so if you are not using expansion voices, the available polyphony for internal voices is 128 notes stereo or mono, no more!
To me, suggesting that the available polyphony is 256 notes smacks of the way that dodgy manufacturers attempt to 'big up' their products and hide the truth about them, and the statement in the page at the link surely suggests that Yamaha are being somewhat less than truthful about this issue, and therefore who knows what else!
I suspect that users of Yamaha keyboards run out of polyphony more often than they realise, especially those with three Right voices (Tyros and Genos). Remember that multi-element voices will use one note for each element currently sounding, and that voices continuing to sound after key release (with sustain for example), will continue to use polyphony until they decay.
I would love to see a real time polyphony indicator displayed on the main (home) screen, but I suspect that this will never be implemented, since it would clearly show everyone the shortcomings in some of Yamaha's products.
Regards
Ian