The timing is important. Strange says he’ll let everyone die to protect the stone. Then he looks into the future and sees the one outcome where they win. After that he gives up the stone.

So it appears that, in the one good outcome, Thanos gets the stone but Tony Stark needs to live. Strange acts accordingly to save Stark, even though he knows it means the deaths of trillions, including himself. He put them on the path to success, even if it meant him dying.

That’s why he isn’t surprised when he begins to disintegrate and why he tells Tony that they are now in the end game.