I know Anne, it's extremely unlikely, but it's not so easy to state the window wasn't opened by an outsider who was already in the apartment. We can't do that, and neither can the police.
They have to be careful, and so far they have been careful to avoid mentioning any of the controversial areas of the case (apart from the 9.15 sighting, and Redwood clearly didn't enjoy that experience).
The window is definitely one area they'll want to leave well alone.
They only claimed that they found the shutters and window open. They cautiously didn't stage anything.
Mr McCann, possibly when he messed up his bedroom shutters, tried to lift them from the outside and realised he could do it (as his shutters were blocked they didn't fall as normally they would do if you don't roll them with the strap).
I don't believe that Mr McCann manipulated those shutters on that night, I even find it totally implausible given the terrible situation (they
really lost a much beloved daughter). Mrs Webster tried (and didn't succeed) when Mrs McCann told her she found the shutters open.
I agree with you there's no proof, but this is a
presumption. And joined presumptions lead at least to conclude what couldn't have happened, contributing for the eradication of some unhelpful myths.
It is certainly not by chance that Mrs McCann, a few days after hearing Moita Flores' comment on the window, wrote on their site that the window could have been a red herring. 3 years after she launched that red herring to the dogs !