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.