Then what does? It doesn't seem like a perfectly ordinary skill for anybody with a mind to try. I think the fact some of the CP9 techniques could fall very nicely under the most solidly established (thoroughly explained) haki types rather reinforces a theory they are all haki than undermine it. How would Luffy even have known about Sanji otherwise? Luffy never looked like a too insighthful man to me, so he probably has to see something to come to think about it, let alone realise.

Tekkai allows one to ignore cannon fire. I don't think any martial arts muscle tension would go quite that far, even if it allowed you to ignore the fist punches of some weakling. Yet it doesn't require a fruit power, only skills and willpower. If that's not haki, then what it is? An identical power for the exact same result but with a different name?

Zoro is so badass you don't really need any proof, of course.