The problem is that we have many millions of people with NO useful skills. They are also mostly untrainable, or they wouldn't have ended up skillless in the first place. In the past, our economy had a place for these people. The future is likely to be different.
And this is exactly the reason people are pissed off about him saying that. The idea that because someone has a manual labor job must because they are stupid and useless is terrible assumption to make. Most people become coal miners because they grew up in a small town with little opportunity. That in no way implies that they can't learn useful skills and become something like a programmer, it says that because of where and how they grew up they never really had the chance. Lack of opportunity does not equal lack of intelligence or lack of passion.
To be clear, I am not saying that every coal miner could be a great coder, but I am saying that there certainly are some that could be. To simply dismiss them entirely out of hand smacks of classism. "They are poor and do manual labor so there is just no way they could ever be smart enough to be a coder."