This is interesting on several levels: For years the College Board claimed there was no advantage to test prep, students could not "beat the test". Now they're acknowledging what most of us believed (and a multi-billion dollar industry trumpeted) all along -- practice and prep helps.
It is unclear if students in this study also used other test prep besides Khan, whether students using other test prep exclusively would show the same level of improvement compared with those using Khan, what bias is in this sample of students who were motivated to study for at least 20 hours, and whether such improvement from test prep is related to the 2016 redesign of the SAT.
Khan is free which is great for students who are self-motivated. Others will benefit from more guided test prep with a tutor.