My guess (and that's all that it is) is that those Dittos are just dittos that were copying a Meltan. Whether or not they are important in some way isn't clear, but I suspect they aren't.
It seems that the way to get a Meltan is to send a pokemon from PoGO into one of the switch games i don't think it matters what, pidgeys might work fine. Once you do that, you get some items, and the Mystery box. When you open the mystery box, then Meltan will start appearing around you, but you're the only trainer that can see it. After a while, the box closes again, and the only way to open it is to trade another pokemon into the switch game.
You can, according to initial reports, trade from your PoGO game into a friend's switch. So all you need to do to get Meltan is find a buddy with the switch game, and you can catch a bunch of Meltans by cooperating with him.
Again, all of this is based on my read of the initial info about this stuff. Switch should be out mid-November, and we'll learn first hand exactly how this all works.
