I also wondered this, Stumpyaura. The DC specifically requested runners to contact him (although he did say who uses 'Strava or similar app'). Why not walkers etc too? So there must be something specific to these apps that is pertinent. Proof of time and location? Why not runners who don't use a fitness app? Weird.
He's given the time frame between 1.30pm to 5 pm initially. Does this indicate L's phone ping in the skateboard park was previous to 1.30pm then? Maybe someone has come forward saying they saw a runner in that area at the time.