I don't think that's true. I believe that adding fins in the 3 to 5 inch range will increase drag, but they allow the board to be pushed harder thereby increasing drive/speed. Perhaps in a controlled environment, where the two boards (with fins and without) where strictly trimming and there wasn't a control issue (the finless gets out of control quickly
), the finless board would have a higher "terminal velocity".
For practical purposes where we're riding/turning/pumping the finned board generates more speed because of control.