Well, I think you disagree on the meaning of the word "artifact." GP cares that all production deployments are identical to the development environment that went through system tests. To ops, a reproducible build is one that has been uploaded to the registry, because it can be ported over to the production cluster and it will be bit-for-bit identical.

It is not necessary to reproduce the same build bit-for-bit because you have kept a copy and distributed it.

You are not rebuilding the same thing twice because that is an inefficient use of resources.

Nobody really cares that the timestamps are different when the builds started at different times of day, it is evident therefore that you will not produce bit-for-bit identical builds as long as your filesystem includes these kinds of metadata and details.

If you built it again, it is to make a change, so there is no point in insisting on bit-for-bit reproducibility in your build process (unless you are actually trying to optimize for immutable infrastructure, in which case more power to you, and it might be a particularly desirable trait to have. Not for nothing!)