I think I know who Exo Stranger was talking to, and they would've told us in Shattered Cycle.
So, time loops. Exo Stranger was in one, right? Who likes time shenanigans? THE NINE. If the Exo Stranger was battling cycle after cycle to configure the perfect timeline that doesn't end in the Dark Future, who would be better to talk to than nine dark matter dorks sitting around in the solar system? Furthermore, if such is true, couldn't the Nine have told her which Guardian to shepherd to commit a better future to time? Plus, come on, who else would she take orders from?
Now, the next big expansion would've been Shattered Cycle, frontheaders on the poster being Mara and Lizzy Bray herself. A, part of the Nine saga; B, Mara has already talked of how long she has known about the nine previously in the story- so why not Exo Stranger?! Even at the very beginning, the Nine had a proven emissary in Xur, so they could've communicated with her through him!
I mean, hey, who else would've known about us like the bozos who merc'd Ikora? We are the FATED WEAPON, meaning they knew from the start that we were the one who was going to save everything.
And you what's the best part? Its not a retcon! Say what you will about our favorite 3 billion dollar implosion, but if we would've let them cook, I'm sure they could've turned 12 year old leftovers into a fine dining experience.