The thing is that they don’t show it as a 19.3 km ‘ride’. They show it as a 19.3 km ‘route’. And the route doesn’t start until you get to the banner behind the falls. Yes, it’s a long lead in (and I truly wonder why they have you go across the new bridge as part of the lead-in, as opposed to going straight to the banner), but it’s still not a part of the route (which is, effectively, the part of the ride that would be repeated if you keep riding and make no manual turns, whereas the lead -in is only covered once during each ride).
There was some discussion in another thread earlier today about putting the lead-in distance into the route selection, next to the route distance and route elevation, in order to give a more clear picture. We’ll see if that ever happens.