I see the point you are trying to make. If you want to be a good defender you have to be strong in all areas. Memo and Jefferson obviously suck on defense because teams can just expose them by putting them in situations where they have to guard on the perimeter with pick n' rolls. The same thing goes for if you are a bad post defender, teams are just going to expose you by putting you in post situations. Obviously in today's game it isn't as bad to be a poor post defender as it is to be a poor pick n roll defender because more teams can run the pick n roll more effectively than posting, but is still going to hold you back from being a complete "defensive minded" big.
Once again, just to reiterate, I'm not saying "Holy crap Kanter is a terrible defender we are screwed". I'm just saying he hasn't really shown enough to be called a "defensive minded" big. I'm not even saying he can't be considered a defensive minded big in the future.
He has not had the opportunity/time to show enough to be called much of anything. Which includes a defensive minded or non defensive minded big.