They may not perfectly control, but they do the job well enough that the error from that is minimal and of less importance than other issues like limited SNP pa

They may not perfectly control, but they do the job well enough that the error from that is minimal and of less importance than other issues like limited SNP panel coverage, sample error, or measurement error (a _huge_ limit on current GWASes and very underappreciated, given how few papers attempt to correct for it or even mention what sort of psychometric properties the available data has). It is certainly not true, as Alex claims, that the results are nothing but systematic measurement error or population structure.