She'd put a fair amount of reputational capital behind the idea that github wasn't like that, so making a public retraction is the honourable move if she genuinely believes she was publically spreading misinformation previously.
She never said GitHub "wasn't like that" as much as that she was doing her best to change the situation from inside of her own company (GitHub). She encouraged women to join GitHub because of the change she was working on creating-- not because it was a utopia at the present time. That was never claimed. (Though your point is still valid that she was "changing her tune" and that is a risk for anyone to do publicly.)
A very puzzling point of view. She claims to have been harassed all along at GitHub while she was promoting it. This goes to show the limitations of the twitter platform for serious discussion. I'm sure she'd have a more logical case to make in >140 characters. Twitter is the easiest platform to vent on, and that is precisely why it should be avoided in such circumstances.
"I regret defending GitHub's culture to feminists for the last two years. I'm sorry to everyone I've hurt in doing so."
"I've been harassed by 'leadership' at GitHub for two years. And I am the first developer to quit."
Worse than having a serious discussion on twitter, though, is tweeting twitter as a source of breaking news.