@Meo From the way it's drawn, what was probably intended was "it's impossible to climb down without being seen by the people in front", judging that there are people in the perspetctive. Presumably, the actions in order would be to climb from an external route (say the window) while police weren't watching, then when the police come back around, using that same window would not be possible without being seen.
Alternatively, it may be just that he climbed upwards from the window, but that climbing down from the root back down to the window would be more difficult depending on the footholds. Or it may be that he used the window (or hatch route) as you suggested as a last ditch effort to escape incoming police, which would mean the police being there would, again, effectively seal his previous escape route.
Either way, though, I agree that it's poorly explained. That said, the intention seems to be that he panicked in using the chimney, because staying on the roof would eventually get him caught. He obviously was not thinking clearly at that point or else he would have remembered the chimney grate too.