Question re: Ishar and Herald Investiture

In the eponymous Oathbringer chapter Binder of Gods, the Stormfather tells Dalinar that before Ishar was Herald of Luck, he was known as the Binder of Gods, and he was the one to come up with the Oathpact.

If this was before the Oathpact and before he was a Herald... then he couldn't use Radiant Surgebinding to have done any of this. (Not that Heralds would necessarily use Radiant Surgebinding ever.) If so, what kind of Investiture was used to create the Oathpact, and/or was Ishar known as the Binder of Gods because he came up with the Oathpact, or was that an epithet he had already earned, and if so, how? The text seems to imply the latter interpretation.

In short, what kinds of Investiture was available to the Heralds at the various stages of Roshar system history?