> In fact, they're typing so much that they can't possibly waste valuable seconds using a mouse. I must be misunderstanding what they mean because that just can't be right.

Yes, you are misunderstanding. First, it's not only about typing but it's generally about doing whatever you are trying to do without unnecessary delays, including navigation between functions, files, windows, etc. Second, it's not just about saving seconds here and there: the end goal is to stay in the flow state, avoiding unnecessary interruptions and context switches. Every time you reach for the mouse and move the cursor or scroll a document you stop thinking at the problem at hand because you're focusing on the motion, and when you're done with the mouse you waste mental energy resuming your previous train of thought, maybe even forgetting something.

> Even if I must look it up in a physical dictionary, how long are we talking here?

This is actually a good example because it seriously disrupts your thinking by forcing you to pause for some seconds and completely focus on something else. I am sure proof-reading a long document in this way is much more mentally exhausting (and slower, but again speed is not the point).