No, my understanding of the page is as follows. If d (width of the window) is large. Small changes in the wavelength create large differences in phase. So if

No, my understanding of the page is as follows. If d (width of the window) is large. Small changes in the wavelength create large differences in phase. So if you have say light in the range 400 to 401nm coming through the window. There are peaks and troughs in that range, but overall there is no effect. Light sources are like that in general, they’re not 450.00001nm but some Gaussian distribution around a point. And within that Gaussian there are peaks and troughs but they cancel out. That was my understanding anyway...