Back when he produced "Batman: The Animated Series" Bruce Timm had always wanted to set the series in 1939, using retro-technology, old-fashioned automobiles, hard-boiled film noir characters, vintage clothing, etc. While he did get to do that on his series, and give it a timeless quality, in this short he has quite possibly gotten to make his definitive Batman story set in 1939 (after over 20 years of working on the character in various mediums).
The hulking zombie that obeys Professor Hugo Strange is speculated to be Solomon Grundy, a very popular villain in DC Comics history.