I think your proposed comparison follows the headline, and the headline doesn't actually reflect the ruling.

I believe that the ruling is focused on the content, not the client. My reading is that if the same-sex couple had (for some odd reason) asked for a hetero wedding site, the web designer would not have been allowed to deny them business.

This doesn't make sense. Why would a Muslim vendor care to do that in the first place? There is nothing in their religion that says only Muslims can eat halal food. It's a completely different scenario.