The fibers in the board are fiberglass-like fibers, which are not absorbant. They are also sometimes added to concrete to make it more resilient to developing small cracks while curing. They are only an asset to the board, not a detriment.
As for your "water test" of the hardibacker, that is irrelevant. Water and mositure is supposed to pass through it, as opposed to repel it. (That is why thinset mortar adheres to it instead of just slipping off of it; it draws the thinset into it for a better bond.) The TILE and grout are supposed to stop the water before it gets to the backerboard. Moreover, wetting the back of a tile will yield similar results. If the Hardibacker became brittle when wet, then that would be a disadvantage, but, as you might have noticed, it does not, which is another one of its assets.
The backerboard is there to support the tile so that individual tiles set together as one unit once the thinset cures. Hardibacker does not shread like cement board under pressure or jolts, but it adheres to thinset just as well. That is why it is reputed to be better than cement board and often is the preferred backerboard. The primary (and perhaps only) significant advantage to ordinary cement board/wonderboard is that you can score and break it with a utility knife, whereas you should use a circular saw to cut Hardibacker.