For sure they should have had food and water as well as more gas. Although more gas might have made things worse as it would've enabled them to travel that much farther in possibly the wrong direction, making their chances of being found even tougher. I don't know if they told anyone where they intended to be, or for how long, but once lost they may have been so far from where they were supposed to be that it may not have helped.
Flares would be a good thing, and if totally lost then I think staying by the snowmobile would be better than wandering away, where they then got separated.
Amazing things turned out as well as they did, considering such a vast and unforgiving environment. I like trees for a reason!