As Starlight and Trixie both enlist the help of Thorax to save both the Mane 6, the Princesses, and before Discord had forced himself into the group to help, Starlight is struck with the thought of enlisting the help of some other friends who could also be of use to this rescue mission. Spanning the reaches of time and space to save her friends from Queen Chrysalis’s hoodwinking of a crime, turning to similar yet different friends along the way.

Even though Mirror Magic had happened way after this initial event, it doesn't necessarily mean that Twilight had shared her story of another world to Starlight before the events of the end of Season 6. So, they'd be starting off as complete strangers and would make sense to have a more mutual meeting in Mirror Magic as well. There is one continuity hiccup where as to place the Main 7 into the timeline of events, either giving them their geodes or not. It doesn't really make a difference other than a potential story split. One where they have the geodes and one without, the without timeline could just lead back into the natural timeline, but the one with the geodes could have a more in depth diverging timeline if it's the case.