Almost certainly it was an incremental discovery process, first cooking and/or fumigating tuna for other consumption, then extending its shelf life by fermentation and/or sun-drying, discovering that evening out the exterior with paste helps extend it, then solving the problem that extended drying leads to hard consistency by shaving off very thin slices, stuff like that. They didn’t set out with “oh we need very thin dried tuna slices for dashi and seasoning” and then invented a whole process for that.