You missed a key classification in this episode. This world isn't the world that Arrow, Flash, Legends of Tomorrow or Supergirl are part of. While we see or hear about Diggle, Oliver and Argus they are different from the ones you have watched on the other shows. On this world Superman is the sole Superhero,
And you're now done with what is generally considered the weakest of the 4 seasons of this show. And while I personally like having an extended epilogue for a season arc (which isn't typical in serialized shows at all), pacing wise they should have either had a longer run episode or pushed aspects of that conflict into the previous episode to flesh it out more.
There are big chucks of this season I really really love, but there are elements that are far less successful.