It's a sort of casual bigotry. Currently fashionable in circles which should know better.

It's snarky self-deprecation, and a concession about the changing demographics of the sf/f community over time, and particularly online. I seriously doubt Charles Stross is actually bigoted against white people.

But of course this being Hacker News, we don't grok humor.

Meh. The neckbeards deserve it.

White, male and old.

Example usage (found randomly by google, likely can find others):

https://www.veterinarywoman.co.uk/2015/02/the-stale-pale-mal...

Interestingly, google's search results indicate the phrase may be (currently) a UK thing (based on the domains of the results).

White, male, and older (or perhaps "out of touch"), with the rhyming indicating that he's trying not to be too serious about it.

Old white men