So he's incompetent. He knew Luffy was in the middle of fighting an Admiral, and he felt of the presence of a dangerous force descending upon the island, and instead of ending his fight quickly, he just continues to muck about, while allowing said force to just incapacitate everyone? But now that Kuma's here, that just means he can dick around a little longer?
Would he have been able to land it from the jump? Star Gun was a counterattack, after dodging a laser from Kizaru, and he had to get close enough to Kizaru to use it, which was something he couldn't do while Kizaru kept running away to try and kill Vegapunk.