There have been enough and more experiments / audit studies showing discrimination in all sorts of markets (even those where there's a seemingly incredibly strong economic incentive to not discriminate).

So, I'm not sure why therapy would be any different -- therapists are drawn from the general population after all.

I haven't looked at the specific link posted in the OP but I read the article a while ago -- it follows the same methodology as other similar studies attempting to identify discrimination in various markets (for another recent famous one, there's the study looking at discrimination in Airbnb that was talked about in the NYT).

So, am not exactly surprised.