iOS won't consider itself connected to a WiFi network until it can hit that domain and get the response it expects, but some networks want you to be connected even though they don't actually give you full internet access. For example, plane wifi networks where you can stream video to your device but would have to pay for actual internet. So those networks will allow the connection for that site and other captive detectors but intercept actual connection attempts.

Whyyyyy... ugh that’s so dumb. That is just someone shooting themselves in the foot for no reason, or maybe some ex employee did something silly on the way out.