Further to this, Apple's WiFi detection is simple, it'll try to connect to:
http://captive.apple.com/hotspot-detect.html
If it does not get a success reply, the captive web portal browser will appear which will redirect to either Meraki's Authentication splash or a customer Radius Authentication service.
So if your walled garden is allowing access to *.apple.com, the Apple device will think that you have access to the internet without authentication and continue as normal. However the user won't be prompted to authenticate until they try to visit a non https site.