Your assertion could be correct 49.9% of the time and still not invalidate the proposition.
Would anyone care about discussing some 'rule' with barely greater explanatory power than flipping a coin? People aren't going to adjust their thought patterns, in practice, for such an improvement.
What is the propostition? I read it as "That which is adequately explained by stupidity is not (or is rarely) malice".
Otherwise it is a not a statement that can be objectively "factual" and is as such not a proposition.
Goodhart's Corollary to Hanlon's Razor: When an exculpatory heuristic becomes well known, it ceases to be a good heuristic.
Quoting philipov here on HN: "Any sufficiently advanced incompetence is indistinguishable from malice."