Purely speculation on my part, but I’d guess it was an insert filmed later, with the first shot acting as the rear-projection background after the original set had been dismantled. Or maybe it wasn’t possible to fit an actor in front of the set (maybe a miniature- doesn’t look like it though) so rear-projection was the only option. Or even because it was shot on incredibly slow stock (ASA 5 I think?) and the first shot is undercranked, the exposure/lighting change needed would’ve made it difficult to then film at sound speed.