Morkus from Orkus
It says half-orcs who are weaker end up dead, but that human blood often gives them cunning (more intelligence) and self-discipline (implying orcs are unable to control themselves) to go far.
It doesn't say the human blood is what gives them the cunning. It says that the human blood gives the the right mix of cunning, ambition and self-control. Orcs are cunning and the most cunning orcs become the war chiefs(from the MM). I think the cunning comes from the orc side, as I have often seen cunning used to describe creatures that aren't necessarily smart, but can still be tricky. Animals are often described as cunning. Had it said intelligence, knowing that orcs have a penalty, I would probably agree with you.