Yes, which means that if say systemd logs where to be shipped to his ElasticSearch instance, he need to configure Journald to log to text files first, and then what's the point of having the binary format?

Yes, ElasticSearch is storing data in binary, and that's fine, but you're not going to ship the raw Systemd binary log to ElasticSearch, nor any other binary logs for that matter.

In fact in the examples he provides both sources are plain text. Syslog-ng and Apache are plain-text logs. He then transfer them to ElasticSearch, where they're store binary, but that's not what anyone is complaining about. The original source should be text, what you choose to do afterwards is your business.