Freeze frame and a magnifier seems
to show a locked back action just as it drops out of sight. Then when we next see the pistol, the slide is fully forward...assuming a new mag.
He did ENOUGH right that it doesn't really matter.
Besides, the article says so....
The impressive thing is how fast, and how well, the gun shop owner reacted, as shown in the surveillance video camera footage below. He'd clearly had some good training in the use of his weapon, and did so very effectively. Note the rapid reload after he empties his first magazine, before he goes to check on the fallen robber.
With Hi Rez and magnifier...you could be right. Doesn't seem to be a locked back slide when it drops out of sight.