Does a lid on a pot safely "seal" soup? My in-laws have a practice in which they make soup with the lid on the pot, turn off the heat, and leave the pot on the stove without lifting the lid until the next day when they eat it.

Their claim is the heat destroys any existing pathogens, and the lid prevents outside contamination, and that the contents are thus safe to consume.

I found this all a bit dubious. It seems to me that if the lid did have such a good seal, there would be a pressure differential giving significant resistance when trying to remove it.

Is this a safe means of short-term room-temperature preservation?
The seal, the pot, none of it really matters.  

You don't sterilize food when you cook it, you pasteurize.  

Even if it was perfectly sealed, there's still baddies that didn't get killed.  Pasteurization brings food bateria to a safe level, not a 0 level.  Once food gets back in the 'danger zone', it can start growing pretty quick again.

This is the same reason you can't cook a sous vide meal and then leave it on the counter forever.