It isn't so much the temperature, but the duration.
Consider what you do with stubbornly dirty pots and pans - you soak them. When you leave them full of water, it works its way into the deposits, softens them and dislodges them from the metal.
The cast iron seasoning is also a kind of deposit on the pot, and not as uniform or as well-bonded as an industrially produced enamel or teflon layer. Leaving the pot soaking in a liquid will weaken the seasoning somewhat, and the water could find its way under it to the bare metal.
Once the water is under the seasoning, it will also need some time. Rust doesn't form immediately. If you're under conditions where it needs, say, 10 hours to form, that would be 20 times cooking a batch of soup, or storing one batch for a day. Re-seasoning every 20 uses is less frustrating than every time, and if you're also using the pot for other dishes in the meantime, or if you brown onions and/or meat with each batch of soup before pouring the water onto it, the seasoning might repair itself during use already, so the rust won't even form.