Well, it's generic in the type of the array element, but it's not generic in that it only works with Java arrays not arbitrary sequences. In particular it's clearly less generic than count (in clojure) or len (in python).
And it is not at all obvious to someone not intimately familiar with the JVM/Clojure that the element type would in any way matter for working out the length of an array. Indeed as far as I'm aware the OpenJVM memory representation for arrays has a length field at the same offset that's independent of the array element type, correct?
So that alength (in the typical case) is in fact 300x slower than len in python and 100x than count in clojure does not seem something that would be intuitively obvious to most people (including clojure programers).