The ArcGIS Server Resource proxy must be used when connecting to ArcGIS Server if you authenticate users through an OpenId/OAuth provider. (App Login, per the first reference link) The proxy user ends up being the user that is reported if using Editor Tracking.
Ideally, it would be nice if the authenticated app user, not the ArcGIS Server user could be used with editor tracking. Apparently, Workflow Manager already has a similar concept. Another idea might be to enable the usage of an alternative tag on the editor tracking user, such as the current concept of "realms". Instead of user@server or user@desktop, perhaps serverUser@realUser could be passed through the proxy?