It is gaslighting even. It is extremely manipulative to be sorry about a thing someone else did, while in fact the the fuckup originated from you. Own it or offer to investigate/fix it.
A decent answer would have been worded among the lines of:
"I am personally deeply sorry to see you go [acknowledge you take their decision serious]. I don't know the details, but if what you describe is true (and I have no reason to doubt it), this is clearly unacceptable and should never have happened [acknowledge the issue is serious and confirm it wasn't intentional]. You feeling betrayed as a result of this is only natural and understandable, and it is Mozilla that should feel ashamed for it coming to this [validate feeling, admit guilt]. I will investigate why this happened and want to find ways to ensure something like this never happens again [show that you're willing to do something substential solbing the root of the problem]. Nobody should have their hard work just automatically replaced by AI, not you, nor anybody else [afirm you're on the same page as them by appeal to general principle]. I know you likely don't want to have anything to do with this now, but I would be deeply grateful if you had a moment to talk about this with me, personally, not as a member of Mozilla but as a member of the community [ask them about help, acknowledging you can't expect any, show that you care about this beyond any purely official duties]."
Of course that means some work, but this is how I would answer such a thing.
A decent answer would have been worded among the lines of:
"I am personally deeply sorry to see you go [acknowledge you take their decision serious]. I don't know the details, but if what you describe is true (and I have no reason to doubt it), this is clearly unacceptable and should never have happened [acknowledge the issue is serious and confirm it wasn't intentional]. You feeling betrayed as a result of this is only natural and understandable, and it is Mozilla that should feel ashamed for it coming to this [validate feeling, admit guilt]. I will investigate why this happened and want to find ways to ensure something like this never happens again [show that you're willing to do something substential solbing the root of the problem]. Nobody should have their hard work just automatically replaced by AI, not you, nor anybody else [afirm you're on the same page as them by appeal to general principle]. I know you likely don't want to have anything to do with this now, but I would be deeply grateful if you had a moment to talk about this with me, personally, not as a member of Mozilla but as a member of the community [ask them about help, acknowledging you can't expect any, show that you care about this beyond any purely official duties]."
Of course that means some work, but this is how I would answer such a thing.