If your only motivation is "it makes it easier to write tests", and there is no other gain, it falls into the remaining case that you even allow for. You're now splitting functions that don't make sense to be split, solely for the sake of making testing easier. And that is bad. A lovely discrete chunk of abstraction is being split across two functions, that you would never call separately, solely to aid testing. And that is bad. That is all this article is asserting with the statements you quote.
No where does the article acknowledge that method decomposition is a valid software practice. It's pretty clear he considers splitting up of functions to be bad regardless of the motivation. Like others have said - if a large function is too complex to test then the codebase is probably improved by splitting it up. That is a benefit of testing and not a downside.
If your only motivation is "it makes it easier to write tests", and there is no other gain, it falls into the remaining case that you even allow for. You're now splitting functions that don't make sense to be split, solely for the sake of making testing easier. And that is bad. A lovely discrete chunk of abstraction is being split across two functions, that you would never call separately, solely to aid testing. And that is bad. That is all this article is asserting with the statements you quote.