Well, there are a number of issues with that piece.
1. The guy doing the demonstration clearly isn't as tall as CB. In the attached mugshot, you can see that 91cm (the height of the window) would be around the height of CB's crotch.
2. As others have pointed out, being that this reconstruction has been put together for GA's documentary hardly makes it impartial. And that guy seems to be making it look beliberately harder than it needs to be. With CB's height, I think he'd be able to straddle the window frame and ease through it sideways.
3. Their assessment that such an exit would have left scuff marks on the window seems very questionable. Unless it's something hard or rough making forceful contact with the frame, normal clothing isn't going to leave any marks.
4. Same with the standing on the bed logic (although I still don't think CB would even have to stand on there), unless the shoe was really dirty, there wouldn't necessarily be any visible mark left. Especially as several people went in and out of that room afterwards and could have leaned on the bed etc, so by the time the police came, the bed condition may have been altered. From a forensic POV, analysis of the bed cover might have have picked up if someone had stood on there except the PJ never tested for that. The bedding was all stripped and washed by the cleaners within 24 hours before it had even occured to PJ to check them. So it's a bit of a cheek for GA to say they would have found something on the bed. If it wasn't for this bungle, who knows, maybe there was DNA evidence left behind that could have solved this case years ago.
Madeleine: How the police ruined the forensic evidence in her bedroom
5. Even with their analysis, they don't state it was "impossible", rather they state it was "almost" impossible. Another way to read that of course is that it
is possible then.
6. The scenario they test is one of a singular person carrying a sleeping child through the window. It doesn't consider whether a sleeping child could have been passed through the window to another person and it doesn't consider that the child could have been knocked out or deceased, which would obviously have made the logistics of getting her out of there much simpler.