How do we know if this hermie is due to stress or if it could have been natural? The seeds where just bag seed so no information there.
You have plenty of info - you're growing it. Hermie seeds are going to have that trait in their genetics. Some strains will turn easier than others. If you've done everything right in your grow (no heat stress, light stress, etc.) and they turned that easily, the offspring are going to have the same tendency.
Something to consider, you can still smoke the bud, but it's nothing compared to what it could have been because the energy went to producing seeds instead of flowers. If you ever grow a regular strain and there's a hermie in there that you don't catch in time, the entire crop could end up seeded (from just 1 or 2 pods that you miss and they drop pollen - especially if you've got fans running to circulate the air).