But when you know how it makes people feel when you make your assumption visible, you understand the need to act like anyone could be the CEO, the developer, etc. Same goes for women devs who at tech conferences who are too often "assumed" to be girlfriends, recruiters or other non-tech participants.
Yes, the problem will solve itself if black people in tech become more common - but:
1. it's not going to get solved if we make them feel out of place in tech by always assuming they're "the wrong guy".
2. let's try not to make the life harder to the very few black people (and other minorities) who are already in tech?
How people feel depends on their own assumptions on why you made the mistake. Say that yours was a perfectly natural mistake based on innate heuristics, and there is no value judgement implied. If this is the way your mistake is interpreted, no feelings are hurt. But if you spread the idea that this is racism and a value judgement is implied, then feelings are hurt, and it becomes a serious issue.
So the way you frame the issue actually ends up creating the issue.
There is the idea that all people are unconsciously racist based on cultural inheritance of the last centuries (imperialism). It is just assumed by everybody that white people are superior in some way. Evidence based reasoning ( few black CEO => it's unlikely to meet a black CEO => this black guy there is not a CEO )is welcome (defensive) argument. Yes, it would help to increase the number of black CEO's, developers, ... But it's only a part of the problem.