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's either currently faced with an unusual situation that doesn't reflect githubs normal behaviour, or she had no credibility in the first place.
At this point, she seems disgruntled about a single issue and is using it as a platform to stir up wider controversy.
Are you like... going back in time and revising your opinion?
"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.