I'm not sure what Lewis is trying to say here, but the physical science meaning and the legal meaning of "law" are different enough that I think it's better to consider them different words that are spelled the same (and etymologically related of course). Which means he's making a pun.
I think it does makes sense to consider them as particular cases of a more general concept, after all. Grammatical rules and the rules of chess would be other instances, somewhere in between.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.