Why does a mixture of siloxene and cerium(IV) sulfate luminesce?
The idea of chemiluminescence is pretty simple, you need an oxidizer and a substance that actually gives a glow.

According to Wikipedia cerium sulfate is rather an oxidizer especially under acidic condition, which actually explains why we need sulfuric acid, so basically, whatever is siloxene (I suspect it is a silicon oxide kind of structure) it is an oxidant that when oxidizes the difference in energy level causes a glow in the visible spectrum.

Unless more clarification about siloxene, this could be the only possible answer.

sources:


NurRage explanation about chemiluminescence
Wikipedia for properties about cerium sulfate


EDIT: what was meant by potential sources is that the method of using words was mine but the information is from the sources sorry for inconvenience