From the article:
” [Her lawyers] argued her attendance at the Met Gala falls under an exception to House rules concerning gifts to members of Congress for “widely attended” events.”
https://www.vogue.com/article/everything-to-know-met-gala-guide
The event usually hosts around 600 attendees.
BTW: The event is more secret than a Bilderberg meeting.
https://www.vogue.com/article/everything-to-know-met-gala-guide
What happens at the Met Gala?
In short: it’s a secret. For this reason, guests must abide by the no phone (and, therefore, no social media) policy.