Maybe you have a funny network situation, but my network situation is solid, and my IP address rarely changes. IP addresses are absolutely identifiable information.

They are SOMETIMES identifiable information. They are not RELIABLE for identification.

If you are going to be collecting information, you are not going to choose unreliable information when you have the option to collect reliable information. That would not make sense.

I can right now open my vpn, change my ip address, go to my apple account and it will report that my Mac is "Online", meaning that apple knows that my particular machine is on with a given IP address. It would be completely useless (redundant at the very least) to also include the uid in the signature file.

There might ten other, a hundred other people with the same public IP visible to Apple, if you ISP is running you through a NAT.

That fact that you have connected from an IP recently in no way guarantees that the next connection from that IP will be coming from you. That is exactly why you need to send an identifier for any data being collected.

Every time you change network locations, all of these daemons contact apple again. There are plenty of push services on your machine and apple keeps track of where you are.

also ipv6

And Apple updates your ip continuously in their server as evidenced in the “find my” service.