I kind of don't see logia types being automatically immune to magic. It's not like they are immune to all damage, just that their bodies can't be hurt by physical attacks since they are made of their element.
It's like how Croc can be hit if there is water involved because he is sand. Light is not immune to magic (especially Sarda's) and Sarda isn't going to attack with physical attacks.
As for being hit at the speed of light. Kizaru is going to at least take a couple seconds, cause he likes to pause before doing stuff, and Sarda can freeze time.
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