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).
This is needlessly reductive and unhelpful. One can work to end racism and police brutality without supporting groups like BLM and/or making public declarations about the issues.
So, no. Silence is not complicity.