> Ivy leagues accept 12% of students from overseas, which will skew the demographics towards the global average.

Only if the 12% is itself a random selection of the global pool, rather than selective by the same kind of criteria the schools use for other students.

The 'Methodology' section of that image (text on the bottom) states it only looks at the fraction of non-international students with a known race. I.e. it subtracts international students and students with race listed as 'unknown' from the total, and considers that the new 100%.