Like the sibling post said, you're probably seeing certificate errors caused by the portal, not traffic being allowed to captive.apple.com.
With http, a captive portal system will intercept your connection and redirect you to the portal authentication page. Most modern devices deal with it automatically by checking those plain http urls when the network comes up. For example, I think the way it works on iOS is that when you connect to a WiFi network the OS tries to hit http://captive.apple.com which triggers the redirect and prompts you for authentication.
With https, there's no way to have a valid TLS certificate for a random site the user is connecting to (ex: captive.apple.com), so you get a TLS error if you're attempting to connect to an https site while the portal is trying to redirect you for authentication.
With http, a captive portal system will intercept your connection and redirect you to the portal authentication page. Most modern devices deal with it automatically by checking those plain http urls when the network comes up. For example, I think the way it works on iOS is that when you connect to a WiFi network the OS tries to hit http://captive.apple.com which triggers the redirect and prompts you for authentication.
With https, there's no way to have a valid TLS certificate for a random site the user is connecting to (ex: captive.apple.com), so you get a TLS error if you're attempting to connect to an https site while the portal is trying to redirect you for authentication.