The article isn't intended to name and shame a particular individual. It's meant to warn women about how gender discrimination can affect their career even if they are very careful to work in a meritocratic team of people who treat them fairly. The claim of the article isn't "this man was sexist" - we have no idea who this man was, and we don't get anything close to the full story. The main point of the article is "this is a thing that happens". I don't think that claim requires extraordinary evidence. It's just an attempt to bring attention to a plausible mechanism by which gender discrimination can negatively affect women, which they might not have been aware of otherwise.
Given that, I don't think we are in a situation where we should bias in favour of innocent until proven guilty. We can just make our own best guess based on what we know.
Your evidence 2 and 3 are weak. Being in a high pressure job often means being in a position where you (and the company) can't afford a big disaster. Lots of people are hard on themselves, it doesn't mean she was on shaky ground because of her competence (her colleagues wanted her around for another year at least). She insisted on leaving to make this guy happy because his stated reason for rejecting the job was her. Her colleagues wanted the guy to join them and they didn't have any reason to refuse her resignation. That doesn't shed any light on why the man was unhappy with her in the first place.
The guy didn't give convincing examples or well thought out reasons, he seems to have just made a guess that she was incompetent. You think his guess is unbiased and the coach and the author's guesses are biased. Even though it is the coach's job to make objective judgements about these things. In this scenario the man, the author and the coach all make judgements given information we are not privy to. Each of them has their own biases, their own agendas. I think the man's inability to explain his dislike when pressed, and the fact that the coach is supposed to provide negative criticism indicates that the coach's judgement is most likely correct.