This isn't the argument actually being made here, so if that's what DHH thought when he shut the thread down then that might explain part of the outcry.
The argument is simply that the behavior everyone claims to decry (e.g. racial violence) starts somewhere, far below violence. "Funny names" turn into off-color jokes. Off-color jokes turn into jokes with a darker edge. Then it turns into "just asking questions", and before too long your organization is acting as much as a group radicalization effort as it is a business.
This still isn't "cause and effect", where anyone who chuckles at a funny name is fated to be building the next Stormfront unless stopped by an intrepid band of culture warriors. But we've known since Ben Franklin that an ounce of prevention is better than a pound of cure, so under the 'pyramid of hate' argument, a company trying to foster a safe workplace for all workers should strive to keep potential racism from turning into a problem by enforcing standards of workplace conduct that don't wait for actual racism.
Just like we build secure software by building security in early into the release process.
Just like we build functional and well-integrated software by building the testing early into the release process.
Just like we avoid pest infestation by keeping the food off of the tables and floors. On and on it goes.
The standard we walk past is the standard we accept. https://www.youtube.com/watch?v=dRQBtDtZTGA