Try the same experiment using salt.

The problem isn’t really about the water getting things wet, more about the salt in it adding conductivity that can corrode metals making holes and also shorting any exposed electronics.

As much as I dislike tesla and it’s unnerving ubiquity along with being under an unstable leader, we have to remember… These are land vehicles, not submarines. They weren’t designed for prolonged immersion in salt water. Most of the environmental testing very likely revolved around using chambers to simulate different weather patterns.

Pressure and immersion testing are generally used only for individual components that do get sealed, permanently. So if you were to seal the battery pack or even just sections, you would still need to connect it all to the electronics like the BMS and in/output. With enough time just these two points could allow a path to short the battery causing the cells to overheat, expand, crack any seals (further increasing the reaction), build enough pressure and eventually pop like a shotgun shells fired outside of a barrel