According to official reports, when Ruby was scurried into the building, the first thing that happened is that he was handcuffed, and then he was taken up to the 3rd floor for processing. It was minutes later that the footage was taken of Oswald, by implication, lying on the floor of the jail office unconscious, and then Ruby was brought back down where he was filmed.
There are a lot of references to Ruby being immediately taken to the 3rd floor. Here's one, from Mel Ayton:
"Dets. L.D Montgomery and 'Blackie' Harrison grabbed
Ruby from the back and got him to the ground. Ruby responded with ''I'm Jack Ruby.
You all know me.'' As he was taken to a third- floor interrogation room, Ruby said, ''I
hope I killed the son of a bitch. It will save you guys a lot of trouble.''