The problem with the argument that "people should be treated equally regardless of other factors" is that we have a long. deep history of that not being the case.

We have numerous studies which how simple biasing factors, such as a person's name, can dramatically decrease interview rates.

Arguing against diversity policies can be coached in the language of equality, but the objective results show that the outcomes would not be.

The outrage, fairly or unfairly (it is impossible to know what's in a person's heart) comes from many readers believing that the person is not arguing for equality, but coaching an argument against it's outcome in language that assumes that goal.

For many people, they see a cheap rhetorical trick made in bad faith.