The term IP spoofing used to really only apply to some networking layer in my experience, placing bogus ips in headers was more likely called header forgery and happened in the application. It wouldn't make sense for wikipedia to rely on easily forged headers when they can simply examine the network connection and use that address.

Actual IP spoofing still can't really impersonate a valid tcp connection unless its all send and no read, even with your second link, both sides of the "tunnel" have to spoof the source ip in their messages so thats not likely going to happen with wikipedia unless their security gets broken somehow and in that case well all bets are off lol

"unless their security gets broken somehow"

yup you get the idea, but you dont have to break security, you can can settle for convincing him to break his own security.