This is probably the same issue that necessitated the doc change in Instruct the user to follow redirects when installing Nix. by SamirTalwar · Pull Request #3716 · NixOS/nix · GitHub (i.e., the macOS install docs didn’t also get updated here).
I think using curl -L may get your collaborator rolling.
For others attempting to follow along, it looks like the underlying discussion about this starts at Manage with nix by jacg · Pull Request #727 · next-exp/IC · GitHub and runs through Manage with nix by jacg · Pull Request #727 · next-exp/IC · GitHub