It's a bit weird. The missions are canon. Like Gibraltar is about pushing a satellite to the launch pad so Winston can activate Overwatch Recall, and the victory for those pushing it is canon. For other maps, it's still canon, the battles did happen, just with certain heroes on certain sides. The game is like if you had a playset with action figures and had them do unrealistic things, like Reaper pushing the Overwatch satellite. So the maps are canon, and there are canon outcomes, but the heroes on each side generally aren't.

The game is noncanon to the universe, its just there to be fun.

The gameplay of Overwatch is mostly noncanon. That's the only explanation.

They want to expand the universe outside of the game to make each hero unique.