But this isn't the same as, say, a child from a non-religious family being required to attend a religious service in school
A child from a non religion/religious belief background would not be required to attend the religious service in school. They have a choice, by law. That’s the point.
If someone is profoundly non religious with a fundamental lack of religious belief, then yes, it may very well be very important to them that the food they are offered has not had any kind of ritualistic religious blessing.
If, as you state: the only issue here is that some prayers have been said out of hearing of the non-believing person then why would them having been said, also out of earshot of the Muslim person, be important to the Muslim person? And yet they are.
If you don't believe, then those words that you didn't hear are literally without any effect
Jews are not believers in Islam, Muslims are not believers in Judaism - Kosher and Halal are not interchangeable for the most observant, I doubt they would agree the words are ‘literally without effect’. Sikhs are not believers in either of those two religions, so the words and ritual having been performed will be very important to them.
The words, and the ritual, is either important or it isn’t. You can’t pick and choose to whom it is or isn’t important based on your own personal opinion.