What you quote is not a commitment to a standard format. It merely documents the behaviour of the current implementation. My statement stands. The format's not standardized. The developers have asserted freedom to change it. I have not seen that recanted. All simple facts. Show me the statement that the format has been locked down.
The current text logs can be read it with less, more, cat, vi or any number of other tools. If the journal format gets locked down, then I should be able to grab any old journal reading tool on any rescue image to read it. That is just not the case today. As you point out, today I must have the latest version of journalctl to be sure I can read the journal on any arbitrary system and that's still not promised to be the case tomorrow.
It's an interesting debate, but I don't intend to carry on. If you don't consider reliably readable boot logs a big deal in any practical sense, that's your opinion. If you just want to re-affirm that you believe the current state of things are acceptable, that's good for you. If you can find a reference that shows upstream intends to lock down the journal format definition, that would be good news and worth knowing.