The fact it’s an online entity may or may not be novel (I imagine Craig’s List has been sued over this).
I meant my comment to refer to the idea of a slippery slope on what does/does not count as a discriminatory. There is lots of case law on that.
I think it’s safe to say if a court has previous rules you can’t put ‘No X’ or ‘Y only’ in a newspaper ads you can use t as a filter for ad targeting.