But no, instead they started attacking the current owner of company publicly (good or bad, doesn't matter, he is now owner of company, if you don't like it just resign professionally).
Imagined another way, if your manager went to Twitter to announce to 1,000 followers how bad a snippet of your code was, he'd be a jerk.
When your boss is abusive towards you like this, it's a good thing to stand up for yourself and/or your peers. I imagine each Twitter engineer responding knew there was a good chance they'd be fired for it.
Now if they handled it the "professional" route you mentioned, that's fine, too. But when Elon started the mudslinging with a lie, I'm not going to begrudge someone telling the truth with some sass.
Its been widely reported that internal communication is ineffective since the takeover, even before the mass firings which drove the expectations to managers have a expected span of 20 professional direct reports (exceeded wildly in some cases) and expected to also be actively coding themselves.
Musk could deal with that breakdown that he caused responsibly, or he could shoot the messenger and reinforce the thermocline of truth effect isolating him from knowledge of what is going on in the company he owns.