Even empirically your dataset consists of startups that target USA markets mostly and hence this conclusion. I am guessing CEOs are good and can come from anywhere, its perhaps "sales" that really needs the idiomatic speak.
I see this happen all the time here at startups in Bangalore. Of course if the audience is YC and the investors, it goes without saying what the language requirement is.
Indira Nooyi (Pepsico, CEO and one amongst that cnn list) articulates it perfectly in this video:
Of course. But again, what does that really tell us? To be effective, you need to be able to communicate with a large variety of people, not just a carefully-chosen subset of people. As Indira acknowledges, it is your own responsibility to ensure this- not your listeners.
I've met people in America with accents almost this unintelligible: http://www.youtube.com/watch?v=I5XyecKONu8 (although typically their mother tongue was of Asian lineage so the sound was different)