Acutally it makes a lot of sense and is no asspull whatsoever.

Lets look what was the orginial idea behind the technique:

Somewhere in the manga it was stated that Kage Bunshin was invented for spying missions with no risks for the user.

Out of that sense there's no avoiding in regaining knowledge or the technique itself would not make sense.

So then an easy example:

Imagine you want to draw a square. You've got 4 edges, so you can start at 4 edges. If you let 1000 people do this exercise and then combine your knowledge you will have at least 4 different ways to draw a square.

Another example:

I think at least the germans here are familiar with this: This is the "house of St. Nichols" oder "Das Haus vom Nikolaus" as we call it. The target of this exercise is to draw the house without painting the lines doubled.

One solution:

I looked it up on Wikipedia: There are 44 solutions to complete this.

So let 1000 people do this and you'll probobly have several solutions.

Redoing it with the 1000 people after they done it the first time will of course increase the result if they collected all the knowledge of all the other people.

So it is working.

The chakra training of Naruto is nothing less, lets make it clear with the example above.

Naruto at 100% chakra makes 5 clones, each having 20% of his chakra.

Given them 5 minutes time,

Clone 1 finds solution 1,2,5,20 and uses 2% of chakra.

Clone 2 finds solution 1,2,6,21 and uses 2% of chakra.

Clone 3 finds solution 1,2,7,22 and uses 2% of chakra.

Clone 4 finds solution 1,2,8,23 and uses 2% of chakra.

Clone 5 finds solution 1,2,9,24 and uses 2% of chakra.

5 minutes later we got Naruto knowing solutions 1,2,5,6,7,8,9,20,21,22,23 and 24 with 90% Chakra left.