May not be true.
US banks have a preference for US graduates and there are plenty to choose from the likes of Stanford, MIT, Wharton.
Singaporeans are quite affluent and are prepared to pay for education.
NUS and NTU business schools cater for the 2nd tier of graduates. A number of them are from the civil service, military and government linked companies. Also some MNCs.
In recent years, China and India provide a lot of post-graduate students. I did a check on the NUS Business School PhD students last year and saw a predominance of students from these two countries.
Anyway, the fault of Lehman Brothers and other US companies do not lie with these graduates. Singapore is only a small blip and their employees do not have that much influence to affect a company of that size. The Americans themselves were responsible for this problem.