That should be overturned on review and called a catch for Stefon Diggs.
To follow up, here's the third-and-14 play where Stefon Diggs felt he was open and Kirk Cousins threw underneath for an incompletion to Laquon Treadwell. (Diggs is the one at the +45 and threw his hands up when he didn't get the ball here.)
Kirk Cousins and Stefon Diggs are sitting together on the bench looking at a tablet. Diggs was open on third down and threw up his hands after Cousins didn't throw it to him.
Stefon Diggs got emotional following his contract extension, saying he kept his promise to his dad.
If it seemed unprecedented, it’s because it was: Case Keenum to Stefon Diggs to end game was first ever “walk-off” touchdown in the 4th quarter of a postseason game, per Elias.