The HP 8460p is about to be superseded by the 8470p; perhaps HP is trying some marketing scheme to move the old stock.
Anyway, unless the extra battery is the same size of the notebook, or they are using it with the CPU in a half-sleep mode or lower, I doubt that it achieves even near that time.
Tests show that it can - but it requires an extended 9 cell primary and an HP slice secondary pack.