The logic checks for a ~/.config/haskell-vim-now/hvn-helper-binaries/stack.yml, and if found considers the binaries to be built and skips further work. However if a previous installation was interrupted then this file will exist and the installer will never make the binaries.
https://github.com/begriffs/haskell-vim-now/blob/master/scripts/setup_haskell.hs#L130-L131