The suspicion of DUI was not the reason he was stopped, but was a result of the stop. He was stopped for suspicion of a firearm, and one that the police did not apparently see, but had been reported to them. Even if the police had seen the firearm, the defendant could have unloaded it and stored it in a locked glove box before the police showed up. Unless they had REASONABLE suspicion that he was committing a crime they had no business stopping him, much less searching him.
Are you OK with being searched because some reports you committing some suspicious level activity?
Non sequitur