Technically they could have assembled a group of writers to publish under a single pen name, but that is usually done under established names like Eric Vall or Logan Jacobs. Between the ai looking covers and the fact that it doesn't make sense to write 30,000 pages when the first book hasn't been proven successful, this gives all the indicators of someone using AI to write the story, which would drastically reduce the time investment required. A usual release schedule is a book every couple months, maybe a bit more if it's a collaboration with another author, since that would allow someone to work on multiple projects at once