There are multiple "speakers" he's not the one and only ever, think of him like a shaman in other games. Grimoire cards say their chosen for their ability to communicate with the traveller. Also osiris didn't like the speaker (obviously as he cast him out because of his obsession with the vex) Osiris believes the traveller is an egg, a womb, for something much much greater. (osiris' theories have been proven correct thus far) Edit: brother Vance also has another motive, the trials of osiris is a charade to make us focus on that. (grimoire card again) Osiris sent him there for another mission, and said not to worry about being discovered as no one will question him due to the trials