Earlier quoted context omitted.
A brain drain means the intelligent population emigrates to other countries. The narrative and data do not support Americans going abroad. I think you're referring to a lack of competitive education for those coming outside of America and choosing Europe / China to study. https://en.wikipedia.org/wiki/Human_capital_flight
I think you’ve got it backwards. MIT used to be brain-draining China, India, Iran, Europe, etc into schools like MIT. The lower numbers mean this is happening less. There are likely multiple factors: becoming less attractive, their domestic options becoming more attractive, more aggressive immigration posture, etc
America losing foreigners in education institutions is not 'brain drain' in the classical sense. There is no emigration (the drain) involved. America receiving all those students and skilled labor over the years was brain drain.