There's nothing implied about tacit knowledge being knowledge that you don't know others don't have. For instance in chess mastery, one is fully aware that you simply see the board in a different way. It's not like you're aloof of your own 'advantage', but then in lessons you work to gradually, step by step, instill such in the student. Some of the most successful teachers are those who themselves learned the game a bit later than average, enabling them to draw more heavily on their own path. For instance Mark Dvoretsky only earned the IM title when he was 28!

I think they're touching on multiple things that typically happens between an apprentice and master, not just tacit knowledge. Also implicit learning, procedural / nondeclarative memory, expert intuition and metacognitive feelings, all very hard to quantify into a written document, but put two people together where one learns from the other, and all these somehow gets transferred over time.