We could spend some time arguing about that, but why bother? The claim is that Kawhi is "hurting the Spurs defense". So the real question is - are those people shooting a higher FG% with Kawhi guarding them than they would with some other Spur guarding them? Maybe we should have Anderson, or Bertans, or Manu guarding the best scorer on opposing teams? I'm sure that would help our overall defensive numbers.
BTW - even the author of that article didn't try to suggest that Kawhi is a sub-standard defender. He's saying that Kawhi's reputation for good defense has caused other teams to use a strategy that takes him out of the mix. What you're suggesting - that Kawhi is not defending those guys as well as the other players they face - that's just an invalid conclusion.