I think it's a bit of misrepresentation to say that Kizaru's stated speed is the sole thing that opposes the idea of FTL speed in One Piece.
I don't know how worthwhile it would be to even bring up other antifeats though. When bringing up the characters being hit by lightning, I was told that it Kizaru uses his
Devil Fruit to enhance to his kicks to lightspeed, AKA increases his combat speed. He does this because in his own words "speed is weight", he is making his kicks more powerful precisely because they're faster.
But now we're being told to accept that actually Kizaru is naturally at least five times faster than the speed of light anyway. Really, his Devil Fruit just handicaps himself and makes him five times slower whenever he attacks...
It's frankly nonsense, and is a very good example of why we should be limiting our headcanon when it comes to these characters.
People try to excuse this inconsistency when it comes to Kizaru by saying "His Devil Fruit is still useful, because it makes his travel speed super-fast! His combat speed is untouched!" But the whole deal with Kizaru's kicks shows how contradictory this is. Evidently his combat speed is not untouched.