From the same source: The overall college enrollment rate for 18- to 24-year-olds was 40 percent in 2020.
https://nces.ed.gov/programs/coe/indicator/cpb/college-enrol...
https://nces.ed.gov/programs/coe/indicator/cpb/college-enrol...
I believe they are actually measuring any time enrollment in that period. Otherwise the statistic seems of little value.
On time high school graduation is achieved by around 86% of the population. This surely accounts for much of the difference.
63% is the statistic for high-school grads, whereas the person you're replying to is talking about all "18-24 kids", so "all kids graduate high school" is an appendix to assumption 1) you would need to add.