It is about time that corporations, with GitHub here as an example, noticed that backing this or that or another minority or "trying" to solve some medial issue only where it suits them PR-wise is simply abusing that minority in yet another way; it is a means of using that minority, and all the people who constitute that minority, as a tool for public relation stunts and political "but we support X, see?" newspeak that brings no actual change.
I'm genuinely curious if GitHub does support Black and Brown people enough to actually make that support noticeable for everyday lives of these folk.
Support by major companies, while admittedly less effective, is another signal that helps. Claiming that it's useless is false and claiming it's cheap is ignorant - the OP clearly demonstrates that a good deed never goes unpunished.
The visibility alone does nil, though, when it comes to everyday practical effects. The mere fact that this issue is visible did not change the death counts. Death statistics during police interventions can be considered one such standard. It's a good first step that needs to be followed by further steps.
In reality, police brutality is nothing new. Politicians should have made changes to keep police accountable years ago, but have consistently failed to do so. Now we've reached a tipping point where the public at large demands change, which needs to come from politicians, and the protests are the manifestation of this.
Does a Tweet from a CEO immediately fix the issue? Of course not. But that's like saying any individual is not going to make a difference when protesting, so they shouldn't bother. Obviously this will only lead to complacency, which will only cause the problem get worse. Democracy, in this case, means as much public pressure as possible so that politicians can no longer ignore the issue without fear of being voted out of office.
Every single bit of additional pressure helps the cause, and whether or not it's also good PR is irrelevant.