The argument against tone policing in a social justice sense is that people who are being wronged by society are allowed to be angry.

This person isn't being wronged. They just don't like the software other people chose to use in their projects which have nothing to do with the author of the blog post!

If someone tells you they're suffering racism you should listen no matter how it sounds to you. If someone tells you they don't like that you're using Slack and that you should not use it, you're right to tell them to sod off until they can be polite.