I believe that includes bluffing and drawing and leading and does apply to single player duels. Maybe things have changed and I don't know how it is now, but I remember reading a couple tourney reports where someone won because they calculated what they thought their opponent had in their hand and believed they could win a counter war after casting a particular threat, meaning they held onto that threat until they felt they were certain they could win.In duels if you play to the best of your ability...
Of course, I've read others where they gambled that their threat wouldn't be countered, but that was usually because they were going to die next turn and HAD to get it out... they didn't have the luxury of waiting...