I disagree, the shooter would have to practice. It might be easy to shoot a couple of people with a handgun at close range but going after large groups with a rifle? Not as easy.
First off if the group comes at the shooter (instead of staying put passively) they could easily be overwhelmed. Secondly swapping out magazines on an AR quickly would definitely require some practice. Firing a rifle is loud, the weapon kicks, it can jam, it is tricky to reload under pressure, if the magazines are fully loaded they can be very difficult to insert etc...
Thirdly the guns were ILLEGAL in California. Not sure why that isn't being mentioned anywhere, I have to think it serves some agenda.
They clearly used 30 round magazines (illegal in CA) and I don't believe for one moment that the guns had a bullet button. Even if they did they surely had an illegal attachment on the outside allowing for a quick magazine change.