The author of that article did not say that the AI did the design of the brand identity. The author of that article said: while I was designing this brand identity, I also used an AI as one of my tools.
Which, to me, reads that Midjourney was a tool he used, not that Midjourney created it. Compare to “How I used Illustrator to design a brand identity”. That’s a perfectly cromulent sentence, and no one would expect that to mean that Illustrator did the work.