Same for the origin IP address. The fiber leaving your country is tapped, and those people can inject packets with any origin IP that they want. Your ISP has no way to check if their peer actually received a certain packet from a certain country or not.

From a technical perspective, all this "china/russia" attribution is built on a quite shaky foundation. As a sysadmin you'd never know if it would be the British crown attacking your European company instead.

Not minimizing nation state cyber crime here, but the packet goes through many hands with different incentives.

Problem here is there are not single fibers attaching (most) countries, but a bunch of them. If you control both the ingress and egress for some particular users it's possible, but if you don't then your probing packing may end up back in China with a lot of evidence of backscatter.