From the main section of the paper published in Nature (which is available for free):

Using AlphaDev, we have discovered fixed and variable sort algorithms from scratch that are both new and more efficient than the state-of-the-art human benchmarks. The fixed sort solutions for sort 3, sort 4 and sort 5 discovered by AlphaDev have been integrated into the standard sort function in the LLVM standard C++ library

It seems they did find improvements for sorting variable sized list but only the sort 3, sort 4 and sort 5 algorithms got implemented in LLVM.