The employee, as a white male in tech, is absolutely morally right to use his privilege to call out other powerful white males for their silence.
And make no mistake, silence is complicity. Many smart philosophers have written about this, see MLK Jr. or Maya Angelou for more.
This is the core of being an ally. Use your privilege to make the hard ask from your peers that a less privileged person, who is decidedly not a peer, cannot.
FB, on the other hand, is also right in a different sense, to maintain internal expectations that singling out colleagues with your political opinion in public is ineffective at best and toxic harassment at worst. FB are signalling to the rest of their employees what behavior they will not tolerate.
In the end, this employee leveraged awareness several orders of magnitude more than had he not been fired (and will likely easily find a new job) and FB protected whatever they believe their culture to be (and whatever other HR lawsuits they believed themselves to be at risk for).
Looking at your profile, I can point out countless atrocities that you don't explicitly denounce. Do I see you upset about how Israel has amped up its program of settler colonialism in Palestine in the past few weeks? How the PRC is running literal concentration camps in Xinjiang? Or, moving along to the USA, how men have extraordinarily high suicide rates? Or how the elderly are being sacrificed at the altar of economic growth in the midst of COVID-19? Or, thinking long term, the tens of millions of people who will die because of climate change?
I don't. And, for what it's worth, I wouldn't be surprised if you have "correct" points of view on all of those. But you're still being complicit in deprioritizing those things and prioritizing your own set of causes, at the expense of human lives. And if you're indeed complicit in a conspiracy of silence on them, you've got blood on your hands.
Brandon Dail was demanding someone add some kind of explicit support for BLM to a Github repo. Where does that stop? I can think of hundreds of very worthy causes that need more publicizing. Is what we ultimately need some long list of evils that every open source project needs to denounce before right-thinking people can choose to use them? And, if you choose to use e.g. Linux, can I denounce you for choosing to use software that is complicit in a conspiracy to terminate black men's lives?
People can prioritize and take action on different causes in whatever way they want to. It's fine to ask individuals to reprioritize, but you're not entitled to anything. And, tactically speaking, ever-increasing stridency of tone and denunciation of imagined enemies is not an effective way to gather support for a cause.