> I said it fixes the same problems as OP listed.

The OP listed noted that hermeticity is required for fast builds, and that humans can't be relied upon to provide hermeticity as a property. How does build2 guarantee hermeticity or otherwise support fast builds without a clean room solution a la bazel?

The OP was vague on why exactly hermetic builds are required to achieve fast builds. The only concrete thing they mentioned is caching which doesn't require hermetic builds (in the strict sense, as in preventing any outside changes) provided you can detect changes accurately.

To give a specific example, Bazel may prevent you from accidentally using a different version of the compiler while build2 will detect that you are attempting to use a different version.

Caching depends on a correct model of the dependency tree. If a dependency isn't included in the model, then a non-hermetic tool will silently succeed, incorrectly while a hermetic tool will do the correct thing and fail the build. Keeping the model accurate is too hard for humans, so in practice, non-hermetic build tools can't cache correctly.