You're still shooting at a target. CJ is claiming(which his claim makes sense), that you are likely to err to on side or the other of where you are aiming your tip to hit the cue ball. So, even though you pick a specific spot in the pocket, you still have a margin of error to make the ball if you mishit.
Have you ever aimed at a specific part of the pocket, made the ball, but noticed you hit another part of the pocket? I'm sure you and everyone else has. So, in theory, you've seen his 3 part pocket in action already.
Now, obviously, it should go without saying when you only have a half pocket or an obstructed pocket to shoot at, you can't rely on a margin of error to still make a ball. You need to take much greater care in shots like these and really hit the ball good.
I don't know why so many people have the same issue with these "systems." Obviously you are going to find shots in which they may not work specifically as 95% of the rest of the shots you will find, but that doesn't mean all of a sudden the system isn't valid.
There is no cure all system for every single shot and scenario on the table, and sometimes you have to figure some things out on your own, or use some basic critical reasoning/common sense assumptions. I'm sure CJ mentions aiming at a certain part of the pocket, but if pocket allowing, give yourself room for error. Even if he doesn't, I mean.....c'mon, why would a world class player who has seen every possible scenario come up a 1000 times, tell you to just shoot in a general area and hope for the best? We can use a little logical deduction and assume you pick a part of the pocket. Even on a basic mathematic level, the "zones" of the pocket are only about the size of a ball, and they overlap each other, as there isn't enough room to fit 3 balls in even the biggest pockets. Sometimes we have to do a little thinking for ourselves.....
How much more logical can you get. Still aim for a specific spot, but if table permitting, give yourself room for error so when you do "miss," you will a lot of times still make the ball. The same principle comes up when you use "pocket speed." When you don't need to slam a ball in to force a cue position, you still hit it firmly, but at a speed in which the pocket will still accept a less than perfectly hit ball. How many times have you hit a ball bad, but because of good speed, you still hit the rail before going in, and the ball still fell? By your theory of assumption used above stating CJ is only using "zones," we should just hit the ball hard and not worry about any need for a margin of error.
If I told you to go across the street for something, but failed to mention to watch out for traffic, would you just run across without looking???
So if CJ says to use a 3 part pocket for most shots(even though aiming at a specific spot), why would you assume he would think a blocked pocket where only 1 ball would fit would still rely on a 3 part pocket??? CJ and most other instructors are going to assume you as a player are smart enough to figure some things out on your own. They only have so much time to cram things into a dvd and it would take massive amounts of time and resources to produce dvds in which they addressed every minute detail such as "a 3 part pocket obviously doesn't exist when there is only room for 1 ball." Besides, if you can't figure some of this out on your own, you're likely not going to be able to get so far in this game anyway.