On the evolution of intelligence bit, he's probably right.
On the enhancement of intelligence, part, he's not... entirely wrong. Given a fixed energy budget (i.e. evolved environment), it seems reasonable that you can't improve the brain very much with gross chemical intervention (which is what he's talking about, in context). Of course, the energy budget isn't necessarily fixed, but it still is interesting.
Surely this depends on what you mean by 'improve the brain'. You might be able to make it better at things you consider important, by undermining things that the evolutionary environment deems important.
Just thought you guys should know about this. Some work that argues that humans should not enhance their intelligence with technology, and that super intelligence probably never evolves.