You forget that high cost producers are typically in China where...

You forget that high cost producers are typically in China where normal rules don’t really apply. You can bet your bottom dollar that the Chinese govt will give their local industry a leg up at the expenses of the rest of the world. So yes in theory high cost producers should drop out but they won’t especially when mines employ large number of people in the provinces (especially around Yunnan).

That leaves the high cost producers in the rest of the world where NCZ is.

Lets go through the large zinc miners (most of them are low-mid cost):
Vendanta have Hindustan Zinc smelters in India
San Cristobal has Sumitomo and Japanese zinc smelters
Glencore has a number of zinc smelters around the world.
Teck has Trail smelter for Red Dog and large longstanding benchmark contracts with Korea Zinc.
MMG have number of partnerships with zinc smelters in China.
Evan Trevali has Glencore has a shareholder which should provide them some protection but not much if the prices drop further.

Remind me again what NCZ has by way of smelter investments?

NCZ will be selling incremental tonnage into a well supplied market without a guaranteed smelter outlet (i,e spot sales). Do the math.