Since it doesn't change the money supply, how can an "expansionary" fiscal policy alone spur economic growth?
In an open economy, Government might be borrowing money from abroad and so the logic of how that capital might have anyway been employed to a  productive end doesn't work out. (Also sale of forex reserves or taking up loans would depreciate domestic currency leading to increased exports and higher AD).

Also, money supply itself doesn't determine the pace or level of economic growth. It is the consumption and Saving patterns that spur demand, leading to the virtuous cycle of increased incomes and investment.

Provided that the C and S patterns are themselves a function of trust and market expectations (especially during times of economic crisis such as recession or slowdown), government through it's expansionary fiscal policy and the popular credence attached to it, incentivise increased activity and growth.