example.com is maintained by IANA. It's an official example address for documentation purposes. So on one hand, it will survive even if Apple disappears, on the other, they're likely not expecting any significant traffic.
Using the site for captive portal access does not actually generate any traffic for the site, because the middlebox intercepts and rewrites the request. Traffic only occurs if there is no captive portal. Hence the easily parsed "Success" body of the Apple site.
Phones confirm whether you passed the captive portal by requesting the usual check url again. That means they'll still get one request after a successful login.