If the screws are removed from the winder the pry it up so its loose and push the window out. If it won't push out then there is an issue with the window as well. Check it's not painted in place on the outside. If so run a blade through it to release the window.
I have seen windows with screws on the outside to also keep it closed (Yes odd but does happen) also paint can cauterize the edge so it wont wont to open.
I was thinking maybe there is a screw under the mechanisim thats covered by the actual winder. But when I went to haev a look I came across this video from bunnings How To Install Window Awning Winders - DIY At Bunnings And it shows how to attach it to a timber framed window, yours will probarly be the same with very little tolerance so that could be why it still feels solidly joined.
