It's amazing that you read the complete overview and still think this is a ponzi scheme. Did you read the math sections, too?
Since the basic income is delivered through upvotes, it is very easy for users to know if they are receiving their expected upvotes or not. If the upvotes stopped going out, no curation rewards, no delegations, no nothing.
In fact, if you study the math (and you can see from the transaction history and upvote history on the blockchain that we are following the proposed math) you will see that there are not any components that even come close to resembling a ponzi scheme.
In a ponzi, somebody sponsors you... that person benefits from you enrolling. In Steem Basic Income, you sponsor somebody else. The person you sponsor doesn't even have to be in the program yet (in fact usually they aren't, that's why it grows so quickly. Okay, that's a a pyramid scheme, which is a simplified ponzi. But still, by definition in a ponzi the people already in the program benefit from new enrollments. In this it's the other way around (read the math section). This is a problem I call 'growth dilution' and I spent a lot of timing thinking about solutions for it.
But you're entitled to think whatever you want. If you have actual logic or reasoning for why you think this is a ponzi scheme, I would love to hear it!