You are entirely missing the point of the graph and data that he is presenting. The percentage accepted has stayed within a narrow band (of 21% or below), while the pool from which they’ve been considered has doubled.
Here is the author in the article says is his point:
“... Thus, according to official statistics, the percentage of Asian-Americans enrolled at Harvard fell by more than 50 percent over the last two decades, while the percentage of whites changed little. This decline in relative Asian-American enrollment was actually larger than the impact of Harvard’s 1925 Jewish quota, which reduced Jewish freshmen from 27.6 percent to 15 percent.
The percentages of college-age Asian-Americans enrolled at most of the other Ivy League schools also fell during this same period, and over the last few years Asian enrollments across these different universities have converged to a very similar level and remained static over time. This raises suspicions of a joint Ivy League policy to restrict Asian-American numbers to a particular percentage.”
To make his claim he needs the Ivy lines on the graph coming to that nice tight bundle around 15% or 16%. Unfortunately for him it is not true so he omitted the last year of data. Additionally, you can not get that number from any of the numbers in the paper. The decline from 20.6% Asian-American to about 16/5% is a decline of about 20% not 50%. Heck using the wrong base only get one to 25%.