The difference to me is that he hasn't admitted his previous views or apologized for the harm he did. Saying, in effect, "Oh, I don't want to talk about helping to strip gay people of their civil rights, but I promise at work I'll be totally awesome," struck me as a) weaselly, and b) unconvincing.
Personally, I don't care much about the donation alone. But his handling of it has been terrible. If he had owned his action and apologized, this would have been a small issue. But from both a staff- and media-relations perspective, leaving this an open issue and trying to dodge the meat of it was a mistake. It's that that makes me think he might not have been a good CEO.
This is close enough to what he actually said:
https://brendaneich.com/2014/03/inclusiveness-at-mozilla/
that it's not at all far-fetched to wonder if there's anything he could have said or done that would have been enough.