I think he means you can't (AFAIK - please correct me) do a hostile takeover of a private company, at least not in the same manner. because in a privately owned company there are no public stockholders to appeal to; the owner and the manager of the business are one and the same.

Really though, what this really is is an example of why you should have a dual-class share system where there are "investment shares" and "voting shares", and investing doesn't give you automatic voting rights. Yahoo for some reason didn't have that, and it put the board especially at risk to hostile takeover.