Yeah, that's the point, as far as you can get with C builds isn't far enough. We're criticizing C build systems here, not nix.

I mean is there a build system not named "bazel" that does a better job of making reproducible builds than a C autoconf project built with Nix?

There's a lot of wiggle room in "better", but there are a decent number of distros for which ~95% of the packages are built in a reproducible way, including Debian, Arch, and Yocto: https://reproducible-builds.org/citests/

And a lot of those packages use autotools; I don't see the C build system as being particularly hostile to reproducible builds.

Yep, and those distributions function just fine without nix. The point is that that's despite the build system rather than assisted by it.