That method of vehicle carry would not be legal in numerous states because the gun isn't actually concealed.
As for Reginald Denny, he might like to have had a gun, but not necessary this sort of vehicle holster for carry. Of course Denny could have avoided the riot situation all together. If he would have had a radio, he would have known the riots were occurring, but he didn't have a radio. Despite the mayhem occurring, Denny did not comprehend what he was driving into when he was driving into it. Once there, he didn't realize what was going on either and stopped his truck amongst rioters despite not having his truck disabled and despite objects being thrown at it and the windows getting broken. He was under attack and remain in place. He was subsequently pulled from his truck, beaten, suffered brain damage, and left on the street. Later, Denny managed to reboard his truck and start to drive off. Several folks came to his aid and one drove Denny in Denny's truck to the hospital.
All in all, Denny showed a rather alarming lack of common sense and failure to recognize the extremely dangerous situation occurring around him and did not try to do much of anything significant to protect himself. He could have driven off long before he was pulled from his truck, but didn't. If Denny didn't even try to do that, then you have to wonder if a gun would have done him any good at all.