One thing that every employee needs to understand is that HR is not your friend. No matter how friendly or helpful they seem, their job is not to protect you. It's to protect the company.
Is there an HR major? Or is it MBA student who never graduate? How does one end up picking a good HR person vs just a friend-of-a-friend? Programmers go to an interviews and they get slammed with whiteboard silly algorithm questions. What kind of question do HR people have to answer to pass the test?
It seems here protecting the company would have been firing the person proposing sex on the spot. Blog author even had the chat logs as evidence from what I understand!
Now the answer is probably they had defended and covered up that manager's behavior before. Blog alludes to that. So it became a pattern. Then just when another similar complaint popped up, admitting it was wrong and doing anything besides what they did before, would have meant acknowledging how wrong they have been in the past. "But we've already covered up for him 10 times, why is this special now..." kind of deal.