This verse is rendered a little clearer in the NIV:
To me this is like the days of Noah,
when I swore that the waters of Noah would never again cover the earth.
So now I have sworn not to be angry with you,
never to rebuke you again.
This is a declaration of the surety of this future promise. This promise is as binding as was the promise that the waters of Noah would never again cover the earth, a promise that the hearers would know had been kept. Because God kept that first promise, they may be certain that he will keep this promise. The “anger” in this case is the cause of the separation, so the promise in the Bountiful context is that of the reunification.