The Nix community has a poor record on security and supply-chain integrity in particular [1] whereas Google has a great record on security, and this announcement (of OSS Rebuild) was written by a member of the "Google Open Source Security Team".
[1]: "it means effectively a decision was made for NixOS to be a hobby distro not suitable for any targeted applications or individuals. It really sucks, because I love everything else about nix design. Instead I am forced to bootstrap high security applications using arch and debian toolchains which are worse than nix in every way but supply chain integrity given that all authors directly sign package sources with their personal well verified keys."
[1]: "it means effectively a decision was made for NixOS to be a hobby distro not suitable for any targeted applications or individuals. It really sucks, because I love everything else about nix design. Instead I am forced to bootstrap high security applications using arch and debian toolchains which are worse than nix in every way but supply chain integrity given that all authors directly sign package sources with their personal well verified keys."
https://news.ycombinator.com/item?id=36268776