My guess.
He studied the ancient knowledge of the Mystics, whose power and knowledge of 'magic' might have been acquired from Lavos. Whatever from the direct contact with the Frozen Flame or passed down indirectly from the relics of Zeal. Perhaps like 'science', magic also has its own self-contained system, which can also be developed and theorized(don't ask me how, I don't know nothing about it
), instead of just inherit. Hence, like we humans have learnt about blackholes and their characteristics before we actually encounter one, the magic users(in this topic, the Mystics) may also learn the mysteries of Lavos(its quality and weakness), although the knowledge may be incorrect or incomplete.
Consider CT is a half fantasy/half scientific story, this theory sounds satisfying enough to me. It is like one tries to figure out where and how Belthasar got the knowledge of time, there may be a short cut for those magic users.
Have been reading Lovecraft for a long time.
How on earth could there be a book which is even older than the history of human?
Where did those cults learn the ritual that summons an Outer God?
Why an incantation has such power which...
Pretty much like such kind of questions. In the beginning was the Word, maybe.