I think there was more than one shooter as well. The old man was a patsy. There is no way that he carried all the contents of the hotel room up by himself. An AR-15 with a bump stock doing magazine dumps is going to overheat the barrel quickly. It had to be a more substantial rifle built for sustained full-auto.
If you watch the complete video the first burst is one continuous burst of way more than 30 rounds. Later you can hear two different guns being fired at the same time.