I'm having troubles wording this, because I know it's obvious, but the killer did exist before February 13, 2017. He had to come from somewhere. So why was he there?
I agree that anyone, from anywhere, can search online and find a trail. It's not impossible that the killer decided he wanted to kill someone in a random, remote place, so drove to Delphi, IN with that express purpose, hoping for an opportunity. It's also not impossible that the killer decided to search for and travel to an old railway bridge, where he happened to get triggered and murder two local girls during his visit. These are both possible, but imo, unlikely.
So where did he come from and why was he there? I'm not 100% convinced he had to live in the community, or have been on the trail and bridge before, to carry out his crimes. But I do firmly believe he had another reason to be in the Delphi vacinity during that time, other than murder. I think he's familiar with the general area in some capacity, for whatever reason, besides just exploring it on a map. He lived a life before the murders, so he had to be doing something, and that something led him there. JMO
It makes sense to me that LE assumes he's either local, or has some family or business link to the area, because it seems too improbable for him to be there otherwise. This is just how I see it.