The increase in heat removal rate for a 20K temperature difference is quite small unless the source and sink have a similar temperature. This emergency exists because they are not even close to being similar temperatures. Evaporative (phase change) cooling does most of the heavy lifting and is very efficient.

It is the same reason sweating cools the body much more effectively in low humidity than high humidity environments regardless of the temperature when you drink it. Attributing how drinking water cools the body via various effects is a classic introductory thermodynamics exercise.

I think you're misunderstanding the tank's temperature. The internal temperature is 32C, the external temperature is only 16C (they used thermal cameras to measure the external temp, but when they got closer they read the internal gauge which showed 32C). Groundwater in LA is apparently right around 16C, so the difference in temperature between the tank and the water would be basically 0.

So while I do agree that the evaporative cooling is probably doing most of the heavy lifting (in fact, necessarily so if the water temp ~= the tank's exterior temp), it's not unreasonable to suggest that using colder water would be more effective.

In fact, the wet bulb temperature there is apparently right around 16C, so they've basically cooled the outside of the tank as much as they can using evaporative cooling alone. They can certainly use it to _keep_ it there, but without something else they wouldn't be able to cool it further.

Presumably if there's an exothermic reaction happening internally then the core will continue to rise in temperature based on the temperature gradient through the material forming in the tank. I would assume (since my understanding is that it's some kind of plastic) that it has a fairly low thermal conductivity, so the core temperature will continue to rise as more of it turns to plastic even as the outside is cooled to the same ~16C.

In the limit if they were able to immerse it completely in very cold water (~0C) then the exterior of the tank would also be ~0C (supposing they were able to access sufficient quantities of water). I don't think that's practical of course, but again I don't think it's unreasonable to suggest that cold water would make a meaningful difference.