Because, assuming he's right about what's going on, the relationship between complexity and time isn't linear (so D's performance relative to Python will grow l

Because, assuming he's right about what's going on, the relationship between complexity and time isn't linear (so D's performance relative to Python will grow less impressive as complexity increases, and at some point Python with NumPy may even perform better), and your example isn't representative of a task that takes long enough for speed to matter.