I assume they included this as a sentence because the two words sound similar.

When the subject and verb are omitted, the remaining object is in the accusative. (book or bread is not a subject, so it's not in nominative case)

It's the same in English, except there is no difference between nominative and accusative most of the time. We would never say He or she?, but rather, Him or Her?

because the actual sentence would be: do you want book or bread?

"A bread" doesn't sound like proper English. Now, "a loaf of bread" does sound right, but I don't think that's what the Hungarian sentence says.