The police, DPP, jury and appeal courts have found Bamber guilty. This evidence of guilt automatically means it is not disputed he exited out of one of the windows.
The only and overwhelming forensic and witness evidence is he exited out of the kitchen window. Supported by Bamber himself saying he knew how to exit it.
What more do people want ? A video of Bamber doing this on the massacre night, from a passer by who just happened to be carrying a video recorder ?