It is thought that spathiphyllum in nature are pollinated by small bees but it is possible that there are other insects that could do it and a seed you never saw developing fell into the other pot.

They do not develop spores.

These plants don't require insects for pollination to occur, and it is more likely that wind currents are responsible for much of this activity in the wild...wind and water. Human intervention can occur with a brush.

These are seed bearing plants, just like other angiosperms. Their 'flowers' are just a little bit different from the norm.