>the touchscreen hadn't the resolution to tell appart which letter you meant to type in
Is that really true? My memory of the original iPhone's touch screen is that it was pretty much pixel-accurate.
The article mentions that the keyboard wasn't accurate enough: "But by early 2006, the iPhone keyboard still didn’t have the accuracy Apple needed to ship the phone." I don't think that means the screen wasn't accurate; all it means is that the original iPhone had a small screen, so the buttons on the keyboard were tiny, and hitting them precisely was difficult. That's why the hit boxes of more likely keys were enlarged.
I think OP is misremembering about the reason for the hit boxes.
The base reason is the size of the keyboard compared with the size of thumbs and the imprecision of thumb typing. Adjusting the hit boxes results in a better error rate. It isn’t because of the resolution of the screen or touch detection.