Apparently he thought the conversation was about whether or not a compiler can understand a type at compile time. The answer is yes. Yes they can. But I'd love to see Elon struggle through the description of why he thinks it's "easy" and why what he said is relevant.
whether or not a compiler can understand a type at compile time
In many / most(?) compiled languages that's because the type is specified at compile time. With interpreted languages that's often not the case, and in that case determining the type can be extremely hard.