Estimate of density at 75 g/cm3 is a result of only a single study done in 2012 based on observed gravitational influence by 33 Polyhymnia on other Solar System bodies. Because asteroid is small and it's gravity is weak too, such measurements are highly imprecise and that's the most likely explanation.

Discovery of actual elements with such a density would have a profound impact on design of compact, low-yield nuclear weapons and potentially even pure fusion ones! But that's just a fantasy. No one will rush to build a space probe to 33 Polyhymnia, because it's nearly safe to assume that the 2012 study is simply an error.