Sometimes you really have to question what professionals tell you if it just doesn't seem right. The battery life probably would not be sufficient for a wheelchair but most definitely would be for an average FIRST robot. Also, the battery is rated as 18 Amp-Hours capacity, not 18 amps supply. Roughly roughly speaking, it means you can continuously pull one amp for 18 hours or 18 amps for one hour, etc. You should look at the spec sheet
http://www2.usfirst.org/2005comp/Specs/batex.pdf for a more specific answer.
Plus, twice the motors doesn't automatically mean twice the current draw. What it does automatically mean, is almost half the loading each, which would mean approximately half the current draw (maybe a little more) each resulting in the total current draw of the drive system being only slightly higher than with only two motors. It is kind of the same concept as walking in snow with stilts or walking in snow with snowshoes. You weigh the same no matter what, but the snowshoes distribute the load better, because they have more area. For a robot, more motors, more load distribution. Less current draw per motor. Only slightly more current draw total.