Earlier quoted context omitted.
For anyone who didn't read the linked article (it's short), it suggests that a key element is careful design of intro courses. Students with computer science experience get a more advanced course that is explicitly designed not to make them more competitive in second-year courses (instead, it starts on material that will re-appear in third-year courses). Students with no computer science experience get their own intr…
How does this level the playing field? Why would this not reduce the overall rate of learning for everyone to suit the needs of the slowest students?
If a "Never saw a line of code until I took this class" sits between two "Coding since 11" students, they're going to watch their neighbors effortlessly breeze through the curriculum. If they don't know those student's background, it's not unreasonable if they come to the (false) conclusion that they're not cut out to be a developer because it's just so much easier for "everyone else" (N=2).
If you "track" your intro classes, you can put the first-timers together and likewise keep the experienced students from falling asleep in class by tracking them into something more challenging