vs.
"YOU ARE A DUMB DOLPHIN BLAH BLAH BLAH"
Massive difference.
The former is a warning rooted in a sincere desire to lead the org/employee towards positive change. The latter is verbal abuse that accomplishes nothing but placating the ego of the abuser.
This is not a random person on the street corner shouting at other random passers by. This guy is in a position of power over the people he is communicating with.
Calling a group of people "DUMB DOLPHINS" on an email chain ... Inappropriate, un professional, dis-respectful all seem like a better way to characterize it.
Its like the reverse of when Prince Andrew described Jeffrey Epstein behavior as "Un-Becoming". There is probably a better way to characterize it.
Partner A was just venting frustration after having a terrible day. Is that abuse? YES! Unequivocally, yes.
There is no excuse for insulting, yelling, or otherwise treating other human beings as lesser than. It's abuse, always, even one time. And frankly if it happens once, it's very unlikely it isn't a repeated pattern of behavior.
Also how would our knowing more about the CEO/employee relationship here improve our perception? The most "positive" thing I can think of is, the employees are so used to it they would tell us something like "oh, that's just CEO, he communicates really passionately". That's not actually positive, though. It's Stockholm Syndrome. [1]
Hold on though. You don't have the full context/relationship between GP and the people who downvoted them. It would be irresponsible for you to jump to conclusions about Hackernews or those commenters based on just a one-line grayed out comment.