The issue was not oxygen for breathing, which as you pointed out, would not become a concern for another hour or two. The issue was that the fuel cells which provided electricity for the Command Module could not function since their oxygen supply was gone after the tank exploded. Because of that, they had to hurry because otherwise they would have to use battery power that had been budgeted as their safety margin for powering them when they reenter the Earth's atmosphere. In short, their electrical generator had failed, and they had to conserve battery power as much as possible. The LM's power supply was independent from the Command Module's, so they could use the LM without draining the Command Module's batteries.
