I agree we don't know how capable open-weight models could possibly pass any reasonable safety testing NOW, but that's about currently abysmal state of AI alignment research, not about what is possible in principle. I don't see any internal inconsistency, to be honest. Since Anthropic does not release any capable open-weight models, it's not their problem. If mandatory safety testing is established, companies who want to release capable open-weight models will work on AI alignment research so that they can pass. This seems to be a good outcome to me.
OK that is a position they could take but my point is that's inconsistent with "Anthropic has never advocated for a ban on open-weights models". What you're describing is a ban on capable open-weights models until some future time.