They can arrest you. The point is that when they give you orders you should follow them, because you almost certainly don't know the extent of their authority as well as they do. Even if you know the law (and are sure about it!), you still don't know what they have seen or heard that might give them reason to suspect you, and thus authority to detain you, even if you're completely innocent.
It's incredible that this seems to be mentioned basically nowhere in the MSM. The 911 callers reported that the vehicle was left in the middle of the road with its engine running and the driver's door open.
A minor detail so I can see why CNN doesn't bother to mention it while trying to stoke outrage.
ETA: it seems to have made its way into a few stories online.