My understanding is that Hoover is a gravity arch dam.
My apologies. You are correct that it is a gravity-arch design, as shown on the U.S. Bureau of Reclamation architectural drawings. However, the drawing shows the that thichness of of the downstream face appears not to be as thick (top to bottom) as a full-gravity design.
The USBR site also describes it as: "A concrete arch-gravity type, in which the water load is carried by both
gravity action and horizontal arch action." This would imply that both the gravity features and the arch features are needed to fully resist the full hydraulic force on the back of the dam, and that removing either would cause failure of the other.
On the other hand, one other commentor may be correct that the chosen design may have been selected to give the illusion
that it was over-designed, to calm the fears of the people about this new and enormous facility.
In answer to another querry, both Hoover and Grand Coulee were constructed during the 1930's. Hoover began in 1931 and Grand Coulee in 1933.