seeding bacteria from old substrate

Tyyrlym

You need to test for ammonia, not just nitrite and nitrate. Ammonia levels will rise long before you ever see any nitrite or nitrates. Ammonia is every bit as lethal to fish as nitrites and you have to manage it. The first step in managing it is testing for it.

Trust me, if the aquarium dried out the bacteria all died. These bacteria will survive without water in much the same way you'll survive without air.