direct from the article in the OP
2) Elite law school grads are more likely to come from affluent families. “When mom and dad are both lawyers, and grandpa owned a factory, maybe it's time to focus on art and travel,” Henderson writes. “In effect, one's inheritance becomes one's safety net.”
3) Law school admissions committees are becoming more focused on undergraduate grades and the Law School Admission Test. “This has fundamentally altered the BigLaw pipeline with students who are (excessively?) academic and lack significant brushes with real world adversity—not ideal grooming for a high stress professional service job,” Henderson says.
4) Elite grads tend to go to the most elite firms where the wash-out rates are higher. Still, the evidence suggests that those who leave the elite firms are moving out of BigLaw, rather than moving down to a less elite firm.
i think these explanations are true to some extent. I'd add that even T14 students who do not come from wealthy families often go into Biglaw with the express intent of staying for 1-3 years to pay off loans before they pursue a public interest career.
Also, while this point is likely not significant, most legal academics only practice big law for 1-2 years. Almost all legal academics come from the T14.
A more accurate measure regarding "performance" would be to consider only T14 students who have a goal of becoming partner vs. students @ other schools; but, that would be really difficult to do.