"How many "pioneering mathematicians" are not one-percenters? If the answer is "99%", then there's no bias in favor of those who do well on the SAT. The article doesn't give that information."

The article doesn't give that information, but unless you believe that being in the top 1% is actively harmful to becoming a pioneering mathematician, the odds ratios of they give for doctorates gives you a good idea, since no one will be a pioneering mathematician without earning a doctorate in math these days. Something like 25% of them have doctorates, compared to the general population which is more like ~1.7%, so simply going by proportion and ignoring the curves in https://my.vanderbilt.edu/smpy/files/2013/02/Ferriman_20101.... and the extreme tail behaviors of these things, at least 0.25*0.01 / 0.017 = 15% of pioneering mathematicians will have been 1%ers. (Take into account the tail, and it'll go up quite a bit.)