BLM platform explicitly calls for an "end to all jails, detention centers, youth facilities and prisons as we know them." So maybe GitHub CEO should have not said anything, but he did and its fair for the public (including GitHub employees) to drive accountability to the BLM platform.
And this says nothing of corporate political spending. From the perspective of a dissenting employee, what is the difference between having co-workers who demand an end to ICE contracts (in response to leadership public supporting BLM platform), and collecting salary from a company that donates to politicians who want to abolish ICE?