What I think Fred is avoiding is the company-specific micro-view. In that case, I think "yes", many late stage startups seem to be over valued, just by applying Fred's yield logic (and growth-risk accounted for). Zynga was one of those companies (and obviously) we never heard how overpriced this was at IPO from Fred.
The valuation of these over valued startups seem to be driven by (i) increase in capital/competition from funds, (ii) eagerness by public markets to jump on the tech/startup bandwagon, and (iii) unrealistic "believe" that double-digit growth is sustainable for years to come.
If the cost of money rises, companies that are only viable while the cost of money is low go out of business, and their employees' salaries drop to zero.