I would assume he's talking about the open valley as opposed to a solid lifter valley. Pontiac cast extra material into their real high performance blocks, aftermarket blocks cast even more material in their blocks to further strengthen the castings.
Using the open valley as a reason to avoid Pontiacs is a bit wide sweeping statement, been a lot of factory blocks that lived in high HP applications for more than just one race. The guys that designed the first Pontiacs did an admirable job I believe, I never thought of the open valley as a huge mistake in initial design. Using factory blocks is always going to be a factor in HP use, but if Freiburger wanted to build a Pontiac luckily we have alternatives for stronger blocks. I believe he's just incredibly prejudiced, as I am too. I don't like chevys fords or mopars. Never been beaten by a buick 455 either....
