Filters/transparencies associated with the lens wouldn't appear white, but black; they could only block light, not create it. (Placing one over the end of the lens itself would be pretty much invisible due to being wildly out of focus.)
If I'm not mistaken, it was an offshoot/sub-page of Modern Followspot Monthly, an Onion-type spoof site. One day both just up and disappeared, as though they had run out of carbons.
The end is very nice. I just think that they were told do some stuff with the light and then on this cue pull out and the center stop on center. Just picking at nits.
The same way Credits were done at the time. It's a peppers ghost arrangement on the front of a camera. A piece of glass is placed at a 45° angle to the lens. A reverse printed negative is scrolled past a linnebach type projector mounted at -45° to the glass. The camera shooting straight ahead sees the image superimposed over the scene the camera is pointed at; varying the intensity of the light varies the opacity of the 'white'. Yes, there are other angles and keystone-ing things that had to be done, but that is the basic. OLD film credits were done the same way and scrolled by hand. you can often see pauses and shakes in the scroll. The Title scene at teh opening of Star Wars was done the same way, shot against a chroma key then overlaid on a starfield.