Note that NixOS and reproducible builds did not detect the xz backdoor, and in fact NixOS shipped the malicious builds of xz (though they didn't do anything because the malware didn't target NixOS):

I am a NixOS developer and I was surprised when the backdoor was revealed to see that the malicious version of xz had ended up being distributed to our users.

As always theory and reality are different, and the thing that made xz possible was never a technical vulnerability with a technical solution—xz was possible because of a meatspace exploit. We as a community are very very bad at recognizing that you can't always just patch meatspace with better software.