Because the mirror technique is literally him traveling along a beam of light he shoots out earlier literally traveling at a speed of light.
And it proves what I’m saying Kizaru doesn’t always move at light speed
Again, you're just choosing to pick random stuff when Kizaru was literally dispersing in particles when creating the clones. Whenever he moves away, we also see a beam of light, plus characters when they are unable to follow his speed.
Luffy often lost Kizaru's sight, guess why?
Snake Man also lost track on Kizaru whilst missing his attack and looking at the direction Kizaru previously was standing.
There were so many occasions when Kizaru was just randomly in some space and threatened Luffy's crew members including Vegapunk. Shit like that never happened with Kaido and this dude is extremely fast as well.
It proves absolutely nothing but if we take that acceleration stuff literally, the only conclusion is that this technique allows him to move faster than light because he already MOVES WITH LS, so what are you gonna pick?
Of course Kizaru doesn't move at LS 24/7 but every technique which involves his DF, mobility and movement is with LS. It was also the same with Enel and I fail to see Kizaru being an exception besides headcanon.