There were three adults along. Maybe the one who walked to the phone couldn't swim.
Anyway, it seems improbable that any of the adults was near enough when the boy fell in for jumping in after him to do any good. If they had been, they'd have stopped the boys from pushing logs into the river ... it's pointless, dangerous, and violates the "leave no trace" rules.
A scout leader who cant swim, should not be watching the boys who are near the water - same thing goes with lifeguards on our beaches - if they cant swim, they shouldnt be in charge of people near the water.