You guys have it all wrong about the CGI Joe
VERY FAKE NEWS
They're right, there's no green screen. Joe's actually there.
It's the reporters who aren't. Joe's just wandering around talking to himself and this was the most lucid few seconds.
They added in the reporters to give it the last touches.
They would be able to control the booms more efficiently if this were the case. He moved past the marker he was given and that's why he went over the boom images that were left to be tracked within a 3D space. Sets have markers where the cameras aren't meant to look and where the actors can see them, but they don't account for slightly more arm movement. He probably stepped slightly over it and moved his hand farther past that.