Their interns are easily the top 1% of CS students in the US. Some of their interns who attend CMU already have OCaml experience, as CMU uses OCaml for their compilers course. Some of their interns have interned at other trading firms.
The OP stuff is brilliant (I guess) but it's small libraries. It's not "senior" work that manages a lot of complexity and deployment risk and $MM cost/benefit tradeoffs, it's hard/smart small work. Exactly what you want smart juniors to do.
Many seniors would never be able to do the stuff in the OP.