There is nothing to specifically rule it out, but I still have a very hard time seeing it that way. Please forgive all the questions, but they need to be tidied up to work that scenario. If we assume HB exited out the back of her house, then we are assuming the perp waited near that exit indefinitely? If she came out the front door, the perp waited by that door indefinitely? If the perp waited in the woods, he ran the distance to her and had just the right view to see her coming out? Did her parents both leave out the same door, different doors? Were their vehicles in the same spot? Did the perp have a vantage point to confirm everybody who exited the home and left in a vehicle? When the parents left did the perp creep up to the exit HB eventually came out of waiting for her? Did the parents come out, go back in and come back out again? If HB decided to skip school for whatever reason, this person was willing to wait indefinitely for her to exit?Maybe he was prepared to wait. I've often thought someone followed her, made themselves comfortable and waited for their chance. They had seen the two adults leave, so when Holly came out, probably assumed the house was empty, not a bad assumption overall.
One scenario that I could see fitting would be if the perp just hid by HB vehicle until she came to it, but then why would they have spent time in the attached garage when we know the car was not parked in it? Lots of questions.
Again, it cannot be ruled out, but there are so many assumptions for that scenario to pan out. Assuming this perp was waiting awhile and was close enough to the home to strike quickly, he would probably need a good vantage point to identify anyone who exited. If the perp was waiting for a female to exit and her mother came out first, would he off approached her thinking it was HB? Maybe this perp was fully aware of who to expect and when? Maybe they even knew where to be at certain times to identify who was coming and going?
Also consider the layout of the house and the surrounding woods. The front door exits to the east, the back door to the west. To the south there are large trees around the home and also looks like bushes, so you would need to be close to the house on that side, it would be hard to stakeout both sides from there. Also on that side, the front door is obstructed by the garage the driveway leads up to. On the north side, where the driveway leads into, the perp would have a difficult time seeing both sides clearly, the swimming pool obstructs the view from the north side and to be at an angle where it does not obstruct your view, you would be so far northwest that you could not see the front at all. Also, that would be a long distance to run from across the house.
All in all, my point is that I feel strongly somebody knew exactly where to be and when, everything else was just fit around that.