Jabber was an option in macOS's messages app, which allowed you to connect to Facebook for example. It was an app to consolidate multiple messaging protocols. iChat (replaced with Messages) was sort of like Pidgin, Empathy, or Adium. However, iMessage, Apple's own chat protocol, never used XMPP. It has always been proprietary. Messages replaced iChat, which only uses iMessage. However, iChat used to technically still be hidden underneath Messages, but was really a separate app. That reddit post is when they removed iChat completely.