You say it better than I do. Agreed it's an extreme selection bias, just want to point out that it's slightly different than the one de Boer seemed to be pointing out.
Yah. It's weird that de Boer argues selection bias is in play in SAT participation between MS (3%) and CT (88%) when 100% of MS high schoolers take the ACT. That seems to be what's happening here: the kids in MS taking the SAT in addition to the ACT are the ones aiming for elite schools.
Agreed. Interestingly, it seems McDonalds ends up being the physical space that plays this role for a lot of older folks, both in urban areas (Koreans in Queens, NYC) and rural areas (Chris Arnade has done a lot of interesting documenting of this phenomenon).