I don't have a SEM link but here's my line of thinking.
I don't think the spouse can start work before the permit is issued or at least not before it has been
approved.
The reunification permit is what allows the employer to hire the person without fulfilling special criteria normally required in order to hire non-EU persons. How would the employer be sure of which rules to follow if there's no permit approved yet?
Without the permit approval, technically the person would fall under non-EU hiring rules.
That's not exactly the same as having the permit issued but I think you get the idea.
Edit - Medea types faster and is less wordy