Considering how fast radishes grow, getting dried radish leaves sounds very feasible.
I would imagine you are correct that much of what people use for seasoning was because of not being able to waste ANYTHING if you were going to survive the winter.
Been perusing Baker Creek on this rainy day. One thing that strikes me having bought bok choy and Asian greens and now surfing other veggie seeds. I've mostly bought tomato seeds and/or live plants. Tomato seeds come 25 or so to a packet. All these other things are 100, 150, 200, 250 seeds. Golden Acre cabbage - 300 seeds. Only 50 seeds for Sapporo Giant and Filderkraut cabbage though.
Baker Creek growers must put a ton of effort into growing picture perfect plants, that and expertise.