@deandddd I wrote a number of my thoughts about this book back a few years ago here: VRIL, The Power of THE COMING RACE
As far as Bulwer-Lytton himself, the only thing I really know about him is that he was a famous English poet who was also a Rosicrucian. It seems to me that being a member of such a prestigious secret society would have enabled him to have access to stories like this regarding the cavern world, and he would thus also have authorization to publish them.
Of course, the cover story for the public for such books is always that they are considered "fiction"—even though Bulwer-Lytton himself never claimed the book to be fiction. Additionally, it was considered an early "science fiction" book even though the genre didn't exist yet.