"Why can't faster typing help us understand the problem faster?"

Because typing is not the same as understanding.

The typing referred to here is not "the typing part of coding" (fingers touching the keyboard), it's the whole coding (LLM is not a typing aid, it's a coding aid).

And coding faster CAN help us understand the problem faster. Coding faster means iterating, refactoring, trying different designs - and seeing what does and doesn't work, faster.