This is where these kinds of issues typically go off the rails. With male-gendered language as the default, I doubt very many people intend to do anything wrong. It's how I learned to speak and I'm sure it's how many others did, too.
A silly nudge from a Slack bot, is just that--not an accusation or a judgment of character.
I was thinking about this a bit more last night, and I thought of another reason why the bot is great: it doesn't discriminate. It doesn't matter if you are in leadership at 18F, a guest user in our Slack, an employee who started two weeks ago, male, female, or something else. The bot doesn't care.
There's a bit of politics to how people get "called out"--e.g. who wants to pull the boss aside and correct their behavior? The end result is likely to be that certain employees get corrected and other employees don't. A bot avoids all of this.