The narrator of the film stated that they took the cinders from the firebox and scattered them by plane over Pike's Peak. Of course that could have been cinders from a previous trip.
It wouldn't have been hard for the 168 to be steamed up. The railroad could have even filled the 168's boiler with steam from one of the other locomotives for its brief trip. I am not saying they did this but it was possible.
The railroad did spend some money for the event, laying down a short stretch of third-rail track.