Static leak at that altitude not really an issue , would only have a small effect , no venturi action on a static , some light a/c have a static source in the cabin for unpressurised types. Cant disagree with the logic of the findings on this system in the report with the information given . Would be of the opinion that a guy of this experience and multiple types and display flying of lighter slower types would be aware of any major altimeter errors visually , so although a posibility don't think its relevant.