[Predictions] Kakashi: Reaper Death Seal

[ShisuiReborn]

I know where you are talking about, and you are misinterpreting it. It's Chapter 124, around page 4. Hiruzen said its the Jutsu of the hero that saved the village. He didn't flat out say that he created it. He just attributes it to him because it was the first time he ever saw it and knew that it was a noble jutsu because its the ultimate sacrafice.

How could Minato create the technique that seals your soul, create a mask to see the Death Reaper, and survive to use it after he created it?

That's all I could find as well. He's just known for using it. The Wiki page explains that it was an uzamaki technique.