That paper also mentions college grades…
Table 2 notes that the 16.5% of intended premeds who fulfilled the premed courses had 3.15 college grades, versus 2.88 for others. This likely means that many of the fulfillers have no realistic chance of admission to a US medical school, based on having GPA of 3.15 or lower, and probably realizes that it would be a waste of time and money to apply.
I.e. fewer than 16.5% of intended premeds will fulfill course requirements with high enough grades that grades will not auto-reject them from medical schools.