^long type hints on let binding values don't do anything:
user=> (def x 1)
user=> (set! warn-on-reflection true)
user=> (let [w ^long x] (Long/valueOf w))
Reflection warning, NO_SOURCE_PATH:13:18 - call to static method valueOf on java.lang.Long can't be resolved (argument types: unknown).
user=> (let [w (long x)] (Long/valueOf w))
but clojure.string/index-of has at least two cases of them, and even if they did do something, there is no reflective code that would take advantage of those type hints.