Of course, linguistic representations won't convey the experiential content, so realizing a truth is different from stating or modelling it. A mystic can say the words "all things are one," but those words are empty and paradoxical compared to the realization or the experience. So the ineffability is a relative thing. My understanding is that genuine mystics tend not to emphasize symbolic representations of deep experience because they think language and reason are bound to mislead and to fail to capture the experience's uncanniness. That's to say the mystical experience is effectively beyond our ability to capture with language or reason. Thus, the experience's content is unknowable if we're thinking of propositional knowledge. (John Vervaeke says there are other kinds of knowledge, such as know-how.)
I wonder whether there's a thin line, too, between realizing something and having only a hunch, intuition, or even prejudice that it's true. Realization might be subjective and thus biased in various ways. Do racists "realize" that some races are inferior to others? They wouldn't know that to be true, in that they couldn't explain exactly how races work or why some are worse than others. But they'd feel it in their gut.
And that's the word I used in the article, which I think you're leaving out: "exactly." Mystics might experience how all things are one, but they couldn't hope to explain exactly how all things, including black holes, distant planets, quantum effects, the human brain, and so are really one thing. You'd need superhuman intelligence for that because you'd need a model of everything in the universe.
Specifically, what I said is, "Suppose a mystic has a religious experience of how everything in nature is united in a divine whole. But the mystic has no map of that unity and thus can’t explain exactly how everything’s really one. The epiphany is ineffable."
You might be interested in a long dialogue I had with a mystical reader on the nature of enlightenment:
