It's a definition. I suppose definitions are tautologies by definition though :)
And yeah, the real insight is not the definition but the fact that tacit knowledge is inevitable and important, and that trying to eliminate it will be actively counterproductive.
I have a lot of habits because of lessons I've learned. There are 100 ways to do it, I chose 1. Documenting the 99 is unreasonable, but that's tacit knowledge, isn't it? I will document why I didn't choose the most obvious one, though.
The key insight (which I agree with!) is that some kinds of knowledge are undocumentable. The challenge - at least for those charged with ensuring processes are documented - is deciding what is and what isn't documentable.