That has always been the core innovation of JIT. Offloading inventory costs onto suppliers makes your books look a lot better. Toyota did make a bunch of other improvements to streamline their manufacturing processes, but the just in time part is really just bullying suppliers into taking on some of your cost and risk.
Partially - suppliers can offset that by increasing their cost to do business; but also most contracts in that world are multi-year deals with ranges of minima / maxima for quantity, time etc.
It also means you can customise your product closer to the delivery time. The vast majority of parts are the same between dozens of final variations of the cars. Having low final product inventory, but plenty of parts stockpiled, means if demand shifts between your final variations you can respond to that quickly while also still being resilient to sudden increases in total demand.
