My PC decided that it was okay for me to record stuff via webcam, so here's a small, boring video demonstrating how weird Ekko's Electrical Storm acts. It's described as being a ranged lightning attack, and yet both Emily and Agnon are immune to it - but for different reasons. Emily's light immune, while Agnon is lightning immune.
You can see that Emily doesn't have her Aegis at this point in the fight, so it's not that. Ekko casts Storm at around the ten second mark.