That's false. The checkbox itself is not a viable undo button under any circumstances in this specific example (i.e., you accidentally clicked but have no idea where, and let's assume you have no idea of that particular checkbox's state prior to the accident). Any adjacent checkbox would have extremely similar plausibility for a user wondering how to undo.

That said, toast is not great either, because it may disappear before the user fully recovers from their accident (say, a spilled drink). Maybe the undo button (and any async success/error labeling for the original event) ought to be adjacent to the checkbox and persist until the next action taken.