I’m not sure what’s crazier to me: asking someone to demonstrate a live proficiency of an abstract skill that is only tangentially related to that actual day-to-day activities of a role or just assuming that because someone has some high credential that they would be good in a given role.
DE Shaw is a financial company filled with maths guys, doing statistical analysis and modeling all day. A math puzzle is the most normal question you could be asked there.
The real question is why Larry Summers is going to a quant interview? Did he apply for a quant role?
For many roles at a hedge fund, being able to do mathematics quickly and intuitively is a valuable skill. Not sure why an economist applying for an MD job would need to be tested on that, though.