He didn't say that the trend would go forever. He said that he didn't see a reason for them to slow down in the near term, we could still be at the beginning of a sigmoid curve.

While it's true that land costs aren't dropping, rural land is still very cheap and due to the nature of solar power it could be almost free.

For example, here in Chile most solar plants are in the middle of the desert. Even if that land had to be paid for, my guess is it's VERY cheap.