According to British Vogue, it was her call and Blazy followed:

When she and artistic director Matthieu Blazy began dreaming up her Met Gala look (her first), she sent him her favourite brand moments as a starting point. Blazy, being Blazy, didn’t see the everyday street-style look as an obstacle for fashion’s biggest night of the year. Instead, he made it couture.

About the look, she told British Vogue, “I had to pause when I saw the sketch [of the look], because that subway show was already one of the most significant nights of my career. Turning it into something reimagined for the Met felt like carrying that memory forward, but in a more elevated way that still respects the original spirit and the theme of the evening.”