This is nonsense.
One game in August does not decide a season even though every game counts. Lefty pitchers have had success against Devers this season. His OPS against them is .704--vs 1.160 against righty pitchers. That's a huge swing.
Also, Seattle manager Scott Servais is now in his 9th straight season at the helm. He had used 4 relievers each in games 1 and 2 of the series and another 4 in game 3, but not lefty Diaz. He had not even used Diaz in the 3 prior games (all wins) against the White Sox.
Plus at that point, bottom of the 10th with O'Neill on 2b and the Mariners having failed to score in the top of the 10th, the odds strongly favored the Sox winning the game.
This was a good time to gamble on the rarely used Diaz who just might, lefty vs lefty, get Devers out. If he did, then righty bat Ref could be walked so that Diaz would face another lefty bat, Smith.
The crucial decision was not whether to walk Devers, but rather was to bring Diaz in to pitch the 10th. We have seen Cora do similar things, which I sometimes call his "dud liberation movement." Instead of letting Diaz continue to hide out in the bull pen, Servais sent him out to show what he could do.
And what he did was lousy. He threw three straight sliders. The first two were barely outside and Devers refused to swing at them, which was a giant clue that he could see those sliders clearly. The third one was right in the middle of the zone, and he clobbered it.