Touch-typing isn't about WPM. It's about the fact that, once you're half-decent at it (and it only takes a few months) the fact that you're typing disappears. So you can write or code without even thinking about typing because the muscle memory turns your verbal thoughts into text.

Hunt-and-peck is more than fast enough (30-40 wpm for skilled hunt-and-peck typists) to crank out code but it's a lot easier to lose the state of "flow" and in programming, Flow is King.

Yes, its important to note here that touch-typing as well as all the talk around VIM and Co. is less about efficiency and more about reducing the cognitive load per step/instruction.

Here's my experience typing on a keyboard (familiar) vs. an iPad (unfamiliar): I can type almost as fast on an iPad, but because of the unfamiliar spacing I have to devote some cognitive energy to the task and occasionally my brain switches from what I'm thinking about (note-taking or programming) to moving my hands to the right place. It does occasionally break my concentration and make it somewhat harder to enter flow. I imagine that I'd have a similar experience if I didn't know how to touch-type.

The problem with iPad keyboards (and all other software keyboards) isn't just that they're unfamiliar compared to physical keyboards, but that they have no tactile feedback.

A touch-typist typing on a physical keyboard will know without having to look that he's pressed the right key and that the keypress registered. On a software keyboard one always has to look.

This makes touch-typing on a software keyboard virtually impossible.

that's because you're phrasing the claim badly. the claim is not typing slowly makes you lose flow, but that hunt-and-peck typing causes you to lose flow.. It's pretty simple imagination from that to hypothesize why that is the the case.