I don't really understand how it is round tripping.

In the end, Nvidia will have OpenAI shares, which are valuable, and OpenAI will have GPUs, which are also valuable. It is not fake revenue, the GPUs will be made, sold at market price, and used, they are not intended to be bought back and sold to another customer. And hopefully, these GPUs will be put to good use by OpenAI so that they can make a profit, which will give Nvidia some return on investment.

It doesn't look so different from a car loan, where the dealer lends you the money so that you can buy their car.