I'd almost argue that, in the example, the clients model wasn't insufficient. If you want to include "implement automated testing" in a small feature request without telling your clien, then your client is probably right to be surprised!
Perhaps I misread it, but I think the only testing being included was for the specific change. The bit at the end was "if we had the time to do a lot of general refactoring and testing not tied to the particular feature being done right now it would save time in the future".
This whole article is just saying "Dear client, here's why your model is insufficient".
Which is a much nicer email to write than "Dear client, here's why my model was insufficient". Which is what happens when the estimates go south