What more can a person do? Just off the top of my head:
He could have addressed the full substance of the blog post, which was a pattern of "intentional, personalized abuse and bullying" with "incidents were numerous, happening over a period of years".
He could have used this incident to reflect on his broader behavior not just toward this one person but more generally.
He could have talked the steps he was taking to make sure he did less harm in the future.
He could have asked for forgiveness.
He could have offered to make amends to Matthew Butterick, to repair the relationship.
He could have recognized that this harmed not just Butterick, but the broader community around the project. He could ask that community for help identifying other times he was abusive. He could apologize for that too and work to make amends there as well.
He could have addressed this bit: "Nor did any member of Racket’s core team hold him to account. Everyone just shrugged and moved on. I was encouraged to do the same." In particular, he could recognize that he was part of a social structure that not only accepted abusive behavior but actually enabled it.
Once recognizing that, he could work to change the system such that abusive behavior was no longer tolerated.
A request for forgiveness indicates that the speaker desires a restoration of the relationship that was damaged. One can have regret ones actions, take responsibility for them, repair any damage, take preventative measures for the future, and still not express any desire to go back to having a good relationship with the person. A request for forgiveness speaks to the motivation of the speaker -- whether it is an interest in the well-being of the harmed party, or merely self-preservation as an individual.
It's considered part of a "four part apology", and is often taught when teaching kids how to apologize.
I'm sorry.
What I did was wrong because ____
Next time, I will ____
Do you forgive me?
Forgiveness is not required (though encouraged), but you should still ask for it. It's not perfect, but following the pattern is something that helps one demonstrate that you understand both why one's behavior wasn't OK (and how it hurt someone else), it also communicates that you intend to change your behavior.
This blog post seems to pretty solidly convey part one, but the others less so. It's still good to see the "I'm sorry".
anglo-saxon culture is shame-based not guilt-based
there's some influence from classic/Med cultures which are guilt based, through greco-roman philosophy or Catholicism, but by-and-large Northern Europe and NA are shame-based cultures and don't really believe in repentance or contrition, but social harmony and restitution
in a shame-based culture asking for forgiveness is largely an empty gesture, and conversely in a guilt-based culture asking for an apology hollows out its meaning and it's seen as a political/diplomatic thing, done outwards for social effect
He could have addressed the full substance of the blog post, which was a pattern of "intentional, personalized abuse and bullying" with "incidents were numerous, happening over a period of years".
He could have used this incident to reflect on his broader behavior not just toward this one person but more generally.
He could have talked the steps he was taking to make sure he did less harm in the future.
He could have asked for forgiveness.
He could have offered to make amends to Matthew Butterick, to repair the relationship.
He could have recognized that this harmed not just Butterick, but the broader community around the project. He could ask that community for help identifying other times he was abusive. He could apologize for that too and work to make amends there as well.
He could have addressed this bit: "Nor did any member of Racket’s core team hold him to account. Everyone just shrugged and moved on. I was encouraged to do the same." In particular, he could recognize that he was part of a social structure that not only accepted abusive behavior but actually enabled it.
Once recognizing that, he could work to change the system such that abusive behavior was no longer tolerated.
And that's off the top of my head.