If a publicly distributed first-hand account by the person in question isn't "solid proof," what exactly are you looking for? What reason do you have to think she's being anything less than truthful?
Her story as far as the article describes, is not an account, it's an accusation without much factual, actionable data. You normally don't judge on accusations until you have tried within reason to obtain factual data from both parties involved.
1) workplace harassment against women is quite common
2) such harassment is frequently ignored
3) there was some harassment in this case
And there's no evidence that I've seen that suggests there was no harassment or that it was handled properly by management. Therefore for the time being my assumption is that the harassment and mismanagement both happened.
That said, although I have seen mountains of evidence for 1) and 2), because the amount of evidence for the specific situation is small, that means a small amount of counter-evidence about the specific situation could easily shift the balance of probabilities.
That said, I have seen quite a bit of evidence that such accusations are only rarely false, so while I would change my balance of properties readily, I don't expect that to happen.
There's even the possibility that she's being completely truthful, but GitHub's side of the situation is relevant and changes the full picture.
You could play the super long odds, everything was AWESOME for the past two years, but just turned awful. Then she was only lying now (because "I've been harassed by 'leadership' at GitHub for two years..."). That would paint her in the absolute best light, and it IMHO, rather unrealistic.
You could even play the super-super long odds, and assume everything she said before was a lie (said it was good, it was actually awful) and then things BECAME good recently and she doubled-down on lying and said things were bad.
... in my case, it isn't an assumption. It is a expectation based on prior behavior.
Here's an example:
X: The economy of Southern Portugal during the first half of the 13th century was bad. Do you agree?
Y: I don't know, I don't have enough info and absolutely zero knowledge of Portugal's economy during the 13th century.
X: are you calling me a liar?
Harry: I can't afford to do that
Tom: I don't know, I don't have enough info here to know if you can afford to do it or not.
Bob: Are you saying that Tom is lying, or that he doesn't know his own situation?
The big difference in this case is that while it is totally plausible for X and Y in your conversation to know literally nothing about Portugal, in my example it is not plausible for Harry to be unaware of his own situation without being an idiot. So when Tom 'withholds judgment' on Harry's situation, he is saying that evidence directly from Harry is untrustworthy - Harry is either a liar or an idiot. (I introduced the third person, Bob, as in this case Tom is Horvath and she is not the one responding to you, doubting Tom).
Harry: John punched me
Tom: I don't know, I don't have enough info here to know if you were punched or not
Bob: Are you saying that Tom is lying, or that he can't recognise when he's being punched?Corroboration. Unless Julie Ann Horvath suffered in silence AND everyone else turned a blind eye, then there will be people who can support her story and provide additional credence to what she says.
> What reason do you have to think she's being anything less than truthful?
Experience. How often have we read a sensationalist-sounding story on the internet[0] that, in the end, turned out to be over-blown or an outright deception?
0 - "Single person claims that large/popular entity has done something despicable/outrageous! More at 11."
This isn't a formal court, but the court of public opinion can (in some cases) be more hurtful and more damaging than a real court that is merely exercising governmental powers.
I am not implying what she is saying is false. As @Jare said, "Her story as far as the article describes, is not an account, it's an accusation without much factual, actionable data."
What's more is her testimony thus far has been very vague. She hasn't described any concrete event.