Many native English speakers can't pick up a pen and paper and write intelligibly and would be in real trouble if they lost their phones; an increasingly annoying number TALK into their phones, not even pretending to type and just spewing auto-corrected crap out into the world.
I'm skeptical of this claim. My family doctor had bad handwriting, and one time the hospital worker had to call him to ask if he ordered a x-ray or Ct scan.
Apparently, it became slightly better recently, now that doctors spend less time scribbling things and mostly type them instead.
What the author describes is a phenomenon like wanting to write the word "analogous" but having no idea how to even begin putting pen to paper. Not writing the word and ending up with "analagous" by mistake.
There are cases, almost certainly overrepresented here, where a character has some truly unique variation that the writer forgets. They know it is different but forget how. In almost all other instances it is a matter of forgetting “is it heart or fire here?” as these two are very similar. It’s like spelling with an i instead of an e.