The IP address appears to belong to whatismyip.com in the past and the code just sends a request to them to get the external IP of which the request is coming from.
So, bitcoin v0.1.0 could only work only when that IP was not down. Anyway, I don't think that it's that important. I don't know if the node saved the connections it received to a log file (so it wouldn't need whatismyip.com), but that version didn't last long.
A GET request cannot send the server any more data than what is in the request headers. And there are no IP addresses in them.
Are you saying that with the whatismyip.com IP, it only took the request headers?
How do you know it's not just one of his ISP's IP addresses used for NAT, that is, grouping a bunch of customers together under one IP address?
You probably can't know who exactly was, but you know that the person behind that IP was a customer. Even if it was a VPN service. Thus, you limit the people that could be. Again, though, you have no permission to ask for such information from a VPN that would be most likely closed right now. Even if we're talking about Satoshi Nakamoto.