Bet there's more than one! Their ability to survive is beyond impressive.

I've had them live in barely damp substrate kept in a bucket in my garage for a year. Like you, I thought it was mostly dried out. Thought it was impossible anything could be alive. Was I ever super-wrong. Most of the ones I have now descended from a handful that were in that bucket.

 

There's retention ponds here in Florida that are usually filled with water but every now and then, they dry out completely.... like this year, there are some that are Bone dry. As soon as they fill, it seems snails come up out of the mud... even though it's dry several inches down! I know Land snails can "estivate" during dry periods, sometimes several years! Perhaps some aquatic snails can as well.
On my recent "Plant Hunt", I collected some plants from Dry areas and put them in containers with water and some snails appeared from the substrate (sand) that I collected with them and their roots. I suppose if there's enough moisture for those plants, there could be enough for the snails.

 

Perhaps some aquatic snails can as well.

For many aquatic snails, they'll survive as long as there's moisture (even if they have to burrow to it) and there's enough to eat.