If you slow motion it through you can see that Milroe threw to a spot to his left, IB's right. He had the DB with his back to him so he could not make a play on the ball if thrown. He threw him open because when he threw it he was not open but there was space that he threw to. No idea if that is what he was thinking but it lined up and Milroe hit the only spot he could hit to complete the pass. The outside receiver on the right side was also open but the DB did not have his back turned to Milroe. Also Milroe was not looking that way so I guess it did not matter that he was open.
Single coverage, DB with his back to the QB, a little space in the corner, Milroe making a perfect throw and IB making a great catch. That is all that had to happen for this pass to be completed

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