Luffy has never used snakeman without FS.
Also I'm not exaggerating the speed difference. Let me show you.
The distance Kaido covered was like 10 meters (Luffy is right next to Kaido after Red Roc, but lets be generous to make the point)
The distance Kizaru covered is around 1-2 kilometers (half a small island-- I'm being conservative to make the point).
Not accounting for differences in reaction speed between forms and just using the distance component of the speed equation (d/s basic algebra), Kizaru would be moving 100-200 times faster than Kaido.
And again, I'm underestimating intentionally.
I don't think you can actually use math in powerscaling like this, I'm just trying to give a sense of how bonkers this feat is