Well, as he goes on to say, a scraping campaign must (based on current law, anyway) satisfy a fairly specific list of constraints to be considered unambiguously legal. That's where the "not clearly legal" idea comes in; I can only imagine that law students would have trouble parsing the idea that a massive tech company with billions of dollars would fail to spend even a little bit of that to evaluate whether or not their scraping was legal and just... do it anyway.