Web AppBuilder integrated in ArcGIS Online can access public and secured services from ArcGIS Server. However, secured services do not support ArcGIS Server with Web Tier authentication, such as IWA, PKI, or LDAP authentication. Refer to ArcGIS Online ArcGIS Sever web services for more information."
Could one read between the lines, and assume that since the author DID NOT explicitly say "Integrated with Portal for ArcGIS", that WAB WILL work with secure ArcGIS Server services, specifically, IWA, if the app is registered with PTL and not AGOL?
I'm running into an issue where the GP Widget in WAB(Dev) will validate the URL for an IWA/HTTPS GP Service, but will go no further than that.