Early use of Zoom lens

Zoom lenses were very rare in 1933. But a scene using one can be found during the shooting of the burning building scene. I was amazed. No other use of it is in the film. Zoom lenses have more elements (peices of glass) in them then prime lenses making them slower also. More light would have been needed. Faster film stocks had not yet been developed yet so most cameramen would not have been likely to use them. How did they do it? Am I the only one who noticed?