I think the sense of "incongruity" meant by the MW definition is "incompatibility."

A collapsed souffle' is perfectly compatible with trying to make a souffle'. You may not expect it, but you know it's a not uncommon event.

But I agree that Oxford does a better job of describing what's usually meant by "irony":

"A state of affairs or an event that seems deliberately contrary to what was or might be expected; an outcome cruelly, humorously, or strangely at odds with assumptions or expectations."

Irony seems like an example of perverse fate rather than a logical or commonplace result.