A quality of most JIT systems I've experienced (and texts on the topic) is maintaining a sufficient buffer (alluded to in your last sentence). Taking it to its extreme (that is, essentially 0 inventory buffer) JIT only works if everything else works perfectly. That is an extreme position not endorsed by anyone or any text I have ever encountered. But it does remind me of the shitty takes on Agile I've seen (no planning, in particular). So seeing it in the wild would not surprise me.

"Sufficient" is dependent on a lot of factors, though. No one can tell you a definitive answer without knowing your system (your suppliers, your customers, your rates of production, your supply of capital to withstand a drop in production).