I watched an old rerun of Law and Order today in which the detectives tell someone's defense attorney that they have his client in footage battering someone, and the attorney's response was:
"No. You have an unidentifiable man in footage battering someone."
And that is exactly the situation with the garage footage. It isn't footage of Ruby shooting Oswald; it's footage of an unidentifiable man shooting Oswald.
There isn't enough visual data there to claim to know that that man is Jack Ruby. And when you start analyzing the small amount of visual data that is there, you realize that it can't be Ruby. He's too short to be Ruby. His neck is too short to be Ruby's. His hairline in back isn't the same as Ruby's. When you stop just assuming that he's Ruby and really try to confirm it, you run into a wall.
There is a reason why the Garage Shooter's face was never captured by any camera, and it's because he wasn't Jack Ruby. It's why they couldn't show his face.