There were several electric cars on fire, and the firefighters used water to extinguish it, only to have it on fire again later. I dont know what is wrong with you, going against facts,...
You are mixing things up and making a wrong conclusion.
It's completely true that it's hard to permanently extinguish a large li-ion pack fire, and it's fully true they easily reignite, but the conclusion that it's because lithium metal is reacting with water, is the wrong one. Your conclusion that fighting them with water somehow caused them to reignite is false, as well. They reignite because they are damaged and store the latent chemical energy and an oxidizer.
It's just that water isn't capable of rendering the cells "safe" in one pass - this is true for all other ways to fight the fire as well. Your idea of burying the burned cells in sand is IMO a very good one, but in practice you would often need to fight the fire with water first, to prevent damage to surroundings. You'd be very lucky to have the fire happen in such controlled environment where you don't need to think about protecting the surroundings. And, stopping the pack fire is the most powerful way to protect the surroundings, you just need to understand it's still not safe after the fact.
It has nothing to do with lithium reacting with water. The
actual reaction is the li-ion cathode intercalation breaking down: consider the most traditional LiCoO2 cathode. More modern NCA and NMC used in most EVs are similar. The issue is that the cathode itself supplies the oxygen, and the cathode stability is poor: it breaks down if enough Li is removed from the matrix due to overcharge, or if temperature exceeds the thermal runaway onset temperature, just around 160 degC for these cathode materials. (A bit more for LMO or LFP.) There is a lot of oxygen in this intercalation matrix, and thus a lot of potential chemical energy; this is
the unstable component of the battery, not li metal, which doesn't exist there in large enough amounts to matter. Additionally, the flammable electrolyte (organic solvent such as a combination of ethylene carbonate and other similar substances) is not helping, and this is the typical source of shooting out flames you see.
The issue is, because the battery has its own source of oxygen (and worse: in a fairly unstable form), it's hard to extinguish: the traditional way of removing air supply (CO2, powder, foam, etc. extinguishers) doesn't help at all; the fuel is there as well; the only way is to actively remove heat. But because the thermal runaway temperature is low, it reignites pretty easily unless actively cooled for indefinite time. Especially because now the cells are physically damaged.
So, understanding the chemistry, it isn't surprising at all that the packs reignite; it's completely as expected. The firefighters need proper training to understand this: a pack doesn't become safe after it has been cooled down and the fire ceased. If this has come as a surprise for any fire brigade, IMHO the EV industry has failed to do their job of arranging proper training for this new tech.
So, water indeed doesn't safely and fully conclude the case, you are right in that. Your problem is assuming that some other way would be better, or that water is adding to the problem.
Water is the only practical way to quickly control the heat release during the incident (which is why it's the officially preferred method of fighting li-ion fire), but you absolutely must know how to do the aftermath. It's not a bad idea to let the cells burn in a controlled environment, that'll eventually release all the chemically stored energy. But, during a firefighting situation, this is often unacceptable - think about a on-board airplane fire as an extreme example. You need to transfer the pack to a safe place first. IMHO, EV manufacturers should take care of this safe aftermath business.
AFAIK, water does not do any such extra damage causing cells to become "more dangerous". It's just that, even water is unable to solve the problem in "one go".
If you look at the official firefighting recommendations, I recommend to look recent ones. The industry and the officials have reached the conclusion of recommending using a lot of water during the last decade or so. We are seeing an evolution here: first they recommended completely random things, not knowing about the chemistry (and li-ion fires were such rare occurences, and packs were small, so it didn't matter much). Then, they understood that you need to fight it with water; but the need for aftermath is still underestimated and poorly specified. I guess we'll see improvement there next.