Soft parts aren't really any different. If you want to make a tire lighter without functional compromise, you're going to need higher-quality stuff. Like a finer casing fabric, or tread rubber compounds that roll and grip just as well but are longer-wearing so that you can use less of it.

It's not like all expensive lightweight tires are just skimping on tread rubber depth. And Rene Herse isn't really in that boat: for a performance-oriented slick, the centerline tread rubber is isn't unusually thin (casing plus tread at centerline is about 3mm when new, and that's with a very thin casing and no puncture-protection layer). And wide tires are longer-wearing just by being wide; my Rat Trap Pass ELs are the most expensive tire in my stable per tire, but because I get around 5000 miles of tread life out of one, some of the skinny road tires that I use are more expensive per mile. That's... part of how I stomach the cost, I guess. But admittedly, I might be going Standard casing instead of EL if black sidewalls were offered in Standard.