Lex had purposefully been cryptic in his wording to attract the two Crystals attention, yet he did not explain what he meant immediately. Nor did he wait for Ezio to respond, for in the given situation Belmont was of more use to him than Ezio. Moreover, based on his behavior of eliminating records of what happened, who knew if he was somehow complicit in whatever scheme was in place which was resulting in the realms degradation.
Therefore, while he wanted to still have a conversation with him to uncover as many secrets as possible, he had to ensure Belmont stayed as well.
"If I'm being entirely honest, I haven't had any time to conduct any real research into the matter myself," Lex confessed. "Anything I learnt was through a series of coincidences. There are some things the sources of which I can share with you, and some that I cannot. It's up to you whether you trust me or not, or find my narrative believable.