rebar3 itself supports reproducible builds via lock files. Applying patches like this instead of embracing the right way that the upstream designed for this exact purpose is just ignorance.
I would be totally cool with any incompatible patches that distro makes for its needs, as long as the binary is renamed. But they shipped a broken binary and called it rebar3. Not "rebar3-nix", but "rebar3". People try using this binary for development and it doesn't work, they go to the upstream and the upstream spends time investigating someone else's hacks.
What I said was that they didn't have a secret agenda. They are completely up front about reproducibility. Code that violates this principle needs to be fixed (or disabled, as in this Rebar example).
If that doesn't meet your needs, then just move on. Nix isn't a distro that will please everyone.