It's very much just a closeup detail of one of her large paintings, which are quite beautiful. The problem is that because it's a closeup and doesn't have much composition, and just the nature of it, when it's applied to the existing Amex card design, it just makes the card look as though it's been battered and scratched up.

Considering Mehretu's paintings sell for 6 and even 7 figures (Wiley somewhat less), I would imagine they got six-figure sums for these designs. Their galleries probably got a chunk of that, and there may even have been some sort of broker involved who also got a chunk.

Here's one of her paintings. They're usually 8-15 feet wide.