Or that this was a relatively nearby civilization, and the light sail was intentionally headed toward our solar system (like a few of our proposed interstellar missions to Alpha Centauri).
That falls under the 'we got ludicrously lucky' category: in the entire galaxy of billions of stars over billions of years we just happened to arise exactly simultaneous with the other civilization in time to see it? (Also, if it was coming from a nearby solar system, why did it pass right through, do nothing, and is at rest with respect to the galaxy? I also think if its trajectory led straight to a nearby star system with any potential for life, someone would've noted that.)