Barring Coutinho and Mane (who arent elite anyway), their whole squad is composed of average players, not just the ones they think are average.
It's amazing they haven't collapsed defensively more than they have this season. Lovren is so inconsistent and Klavan is just a guy. Matip is pretty good, but he really shouldn't be the best CB on a top 4 team, and then you've got underperforming keepers (Karius might come good in time, but obviously hasn't been great and Mignolet is who he is, classic Howard/Foster midtable keeper who often does great and terrible things and isn't great with his feet or at crosses), James Milner at LB and the DM is Henderson, who has played well but is hardly Makelele out there.
Klopp's pressing minimizes their defending. He's a very good manager, and in some ways it's easier to see them winning a title than a side like Spurs because while it's sort of tough to see Spurs improving easily, you could see how Liverpool could sign a midfielder, CB, LB and keeper for 25-30M at each spot and make clear upgrades, if they can spend wisely like they did with Firmino and Mane. They don't need Chicharito diamonds in the rough or Pogba style massive buys to improve (though obviously that would help). Some reasonable, efficient moves could really improve them next season.