Breaking spokes going up a fire road is not common. Did they say why? Just think about the torque/stress through the rear spokes when a miss-timed naughty gear change is made to rescue a blunder going up a steep bit with maximum assist to your maximum effort....
If they break commonly on fire roads, then what do they say about more stressful moments?
The LBS comment sounds fishy to me. Having said that, when me or a mate busts a spoke, it’s always been traceable to a previous rock strike — just my experience tho’....