All of this happened after Kizaru sent Luffy flying across the map, and Kizaru was the one dominating the fight so far. I doubt Lucci thought Luffy'd lose ten seconds into the fight. And I seriously doubt they have time to watch how Kizaru's struggling with Luffy when they're busy with each other.
He doesn't need to witness the fight first hand. The fact that it is still going is telling, especially when Lucci's confidence that they would win with no problems went through the roof when Kizaru showed up.
Lucci didn't recognize Luffy as a Yonko, even after fighting him first hand. Finding out one other, single crew member, is way stronger than he initially thought isn't going to be the tipping point where he goes "Oh, OK, definitely a Yonko caliber crew here."