You failed to understand the study. The 1 case is the marginal effect of a year of education on students who were about to drop out of school, which is a very special type of population. 5 is for an additional year of compulsory "mediocre classroom" education on the entire population of students. Is your failure to understand due to genetic inability (highly unlikely) or due to your strong priors influenced by racism in development environment forcing you to conjure up a favorable interpretation for your argument (much more likely)?

The Lothian Birth-Cohort studies were not designed to measure genetic differences in mental ability by race, and your article does not claim it was nor draw any conclusions in that regard. It merely shows a correlation of 0.66 between mental ability at age 11 vs. age 79, which leaves a lot of variability to be explained by environment after age 11. Multiple studies have shown that intentional changes in educational environment cause large differences, with Bloom's 1-on-1 educational coaching causing the largest differences of two entire standard deviations, the difference between median and genius.