Like all things of the Shadow I think Elza's knowledge makes little sense. I think the knowledge of the weaves was put into her brain, much like Fain received knowledge early on in the series. Fain considered it to be pain beyond belief, so I suspect that SH gave her knowledge but that it was extremely traumatic. For all we know the DO knows everything about the One Power - he certainly knows about balefire, as he told Rahvin.