Maybe I'll add to the confusion here
but I think the problem is that aloe vera is not one compund. When you talk about glycerine then that is a single compund which is clearly a humectant. Aloe vera is a naturally occurring product that contains a whole range of compounds e.g. acetylated
mannans, polymannans, anthraquinone C-glycosides,
anthrones and
anthraquinones and various
lectins (i copied that from wikipedia but it sounds about right). So some of these compounds could well be humectants but the others could be having completely different effects so using aloe vera is not the same as using a typical simple humectant. Or to put it more simply, it's more like using a conditioner made up of multiple ingredients with various functions.