Does eduroam use a captive portal for authentication?

No. Web Portal, Captive Portal or Splash-Screen based authentication mechanisms are not a secure way of accepting eduroam credentials, even if the website is protected by an HTTPS secure connection. The distributed nature of eduroam would mean that many different pages, languages and layouts would be presented to eduroam users making it impossible to distinguish between legitimate and bogus sites (even a consistent layout can be mimicked by an adversary).
eduroam requires the use of 802.1x which provides end-to-end encryption to ensure that your private user credentials are only available to your home institution. The certificate of your home institution is the only point you need to trust regardless of who operates any intermediate infrastructure. Web portals require you to trust their infrastructure as they receive your password in clear text, this breaks the end-to-end encryption tenets of eduroam.