What is MIT, what the subscription pays for, and the one part of this that has not been decided yet.
The hosted service that answers --idea may end up MIT in the same repo, source-available, or closed. That choice changes what this page says, whether a self-hosting guide exists at all, and whether AI_PROJECT_BOOTSTRAP_API_URL is documented as a supported route. This page is reserved in the nav deliberately rather than filled in with a guess.
Until it is decided, the site says exactly this and no more. Claiming the whole project is open source while the paid path runs on unpublished code would be the one dishonest sentence on an otherwise plain-spoken site.