If the window strip were actually quite small, something like 50 cm wide, then I would be with you on the basement idea. As it is, I just can't really understand it. It's just as wide as the terrace door and 60 cm high. I don’t imagine it being that dark. But a change wouldn’t be possible anyway.
It is the parapet height of 175 cm (finished floor probably rather 160 cm) that creates the basement feeling. Have you seen it live? And, a clear opening height of 60 cm will probably result in a glass area of 45 cm height. Yes, some light comes in, but you can't really see through such a window.