In _lookup_ge, where a loop "looks for an available edge or val within the current bisection node" (to quote the code comment), the value of index has already been modified to guarantee that it is the least value than can be found in the non-NULL child node being examined. Therefore, if the non-NULL child is a leaf, there's no need to compare 'index' to anything, and the value can just be returned.
The same is true for _lookup_le with 'most' replacing 'least'.