Perhaps you've gotten use to keeping your hips tilted to one side over many years. That might need a PT to fix.

You can do what the fitter suggested. But I'd want to find out what's really going on. A far back as I can remember, you are the first person to ever suggest their sit bones are uneven. So I'm going to assume that's not the issue until you can prove it with a medical diagnosis from a orthopedist. .