Auth.js is not made by Vercel. Quote: “ The Auth.js/NextAuth.js project is not provided by, nor otherwise affiliated with Vercel Inc. or its subsidiaries. Any contributions to this project by individuals affiliated with Vercel are made in their personal capacity.”

You're not wrong but it's a very understandable mistake when the footer has a big old "powered by Vercel" logo and the creator works for them.

The logo is probably just supposed to another opportunity for them to grab mind share by hosting the site for free and sticking that there, but it creates a confusing impression.