I think you misunderstand the way the Higgs mass is calculated here. This is a highly non-linear relationship that is extremely sensitive to the top quark mass. In fact the second most massive quark would only contribute a fraction of one thousandth to the result (which is why it was ignored in the paper).
Consider this toy example: The mass m is calculated from some parameter like m~=b^4 and that parameter b was measured b=2.0±0.1. Using Gaussian error propagation, m would be 16±3.2. Now update the measurement of b just slightly to b=2.1±0.05. That 5% change in b changes the result to m=19.45±1.85 -> more than 20%, and with just one standard deviation of difference in b. The relationship is not linear.
