JK Galbraith Quote: How to interpret?
Here is the quote, rephrased:

This is a false statement: (A) the poor don't work because they have too much money.

This is also a false statement: (B) the rich don't work because they don't have enough money.

If you believe those false statements (A) and (B), you are very credulous.

The poor have a much higher marginal value of money than the rich, so their incentive to work, to earn more money, is very high. For the rich, it's the other way around.

Ken is making explicit two assumption that are fairly common (in that school of thought and some other Conservative groups), but rarely spoken so plainly, and rarely spoken together. When they made explict and are put together, it makes it much clearer that to hold both of them at the same time is absurd.