"Educational Attainment" and the Wobbly Null

New study related to genetic studies of Educational Attainment:

Avoiding dynastic, assortative mating, and population stratification biases in Mendelian randomization through within-family analyses

Like a previous study, it makes the point that within family analysis significantly "attenuates" the educational attainment, in this case related to correcting for height. Here's the rub, though. In the previous study, the fact that EA was significantly diluted by within family results was somehow lauded as a demonstration along these lines: "At least there is still something, so it proves there is some genetic component to EA." This study, however seems to take the opposite approach:

The Mendelian randomization estimate using the sample of unrelated individuals implied that each 10 cm increase in height caused an increase of 0.17 (95%CI: 0.14–0.20, p-value = 8.5 × 10−26) years of education. After allowing for a family fixed effect, the Mendelian randomization estimate was greatly attenuated suggesting little evidence of a causal effect of height on education

In this case, the attenuation was taken as evidence of a null value, to demonstrate that they were able to get the pop strat out of the picture. However, if something like height has even a small effect on EA and the tiny results for genetic variants for EA after within family analysis, then it's worth asking whether there are any actual genetic variants related to someone being better or smarter in a way that allows them to get more education (c'mon, this should be obvious), or whether there are just a few physical confounders giving us the slight variance accounted for. You can't have it both ways.