Without going into a long drawn out explanation I will say this. Reptile biology is very different from mammal biology. It was very surprising to find out how little is actually known about reptile medicine in general. Combine that with the differences in species and you are left with a lot of question marks.
The biggest indicator to me that makes me agree with the vet is the fact that the animal has not eaten. You stand a higher chance of the antibiotics being effective when the metabolism of the animal is higher. This means that the temps are a little higher than normal and that the digestive process of the animal is engaged. Deborah is absolutely right that this animal is not a baby and it won't hurt it to miss a few meals, however this reduces the effect of the antibiotic. The lower the metabolism the longer the antibiotic course needs to be.