I think it makes it to calling open_exec but there's a test for BINPRM_FLAGS_PATH_INACCESSIBLE, which doesn't seem relevant since 'bash' isn't like '/dev/fd/<fd>/..', but does provoke an ENOENT.
env bash is all well and good for normies, but if you're already on NixOS did you know you can have nix-shell be your interpreter and back flip into any reproducible interpreted environment you like?
Or any other system with Nix installed. I use this at work to provide scripts with all their dependencies specified that work across any Linux distro & MacOS. First execution is slow since it has to fetch everything, but after that it's fast and just works.
I don't have fish installed and can't be bothered to go that far, but I suspect they're right about that as well.