Earlier quoted context omitted.
In my experience it can be a bit of a canary. People who are assholes tend to be extra-assholey to out-groups (which often is minorities). If members of minority groups don't want to work somewhere it usually means that the people who work there are assholes.
You haven't had enough jobs to make a statement like that, no one has. Your effortless translation of "90% white" to "assholes" is a bit of a tell. For instance, you're white, and used to thinking this kind of trashing a group of people is ok because of your socially dominant position. Because these whites resemble you physically right? But of course you mean the bad whites; you're one of the good whites! It isn't ok…
Since racial heterogeneity is a more easily observable situation than, say, diversity of opinion, it's a healthy sign of a tolerant culture. That doesn't mean that a workplace lacking racial heterogeneity is bad. It simply lacks the canary. (If I'm interpreting GP properly.)