Yes agree this is a possibility, however there is no mention of anyone else on the FB post, no photos either of anyone else. there is only one photo of SD on bridge overlooking Mundaring Weir. Considering SD's age group, I would imagine, there would vey likely be pics of friends or tagging of friends, relatives etc.
I assume the conjecture over whether he was on the trip to mundaring is related to this being a possible SS site. I think it's a long shot. Not that we are sure but it seems that the killings may have stopped when he met his wife and became a step father. From what I've seen in his strong relationship with his SD it's quite possible he'd want her as far away and shielded from what he is alleged to have done in the past. It's possible SS is in that area for sure but don't think him going on a trip up there with family is a strong indicator one way or another.