I remember this keenly and am just as salty about it as the author. It is accurate. This was the observed behavior as people's accounts were gradually migrated

I remember this keenly and am just as salty about it as the author. It is accurate. This was the observed behavior as people's accounts were gradually migrated to Hangouts from Google Talk. I remember testing this myself - the Google Talk desktop app would still show conversations from XMPP contacts (and still worked for a time after you were migrated to Hangouts in Gmail), but the Hangouts pane in Gmail would not. Nonetheless, the Hangouts users would still appear online (I believe in "away" status) to the XMPP users.