Not necessarily. There have been instances where a person was able to conceal a gun through being taken into custody and transported. Getting out of cuffs, or out of hands behind the back can be done by "flexible" folks.
That I didn’t know (of course I wouldn’t since I’m about as flexible as a brick). :)
Since the official report says nothing about him getting the cop’s gun, I’m leaning toward a gun being planted or the perp somehow snuck one into the car with him.