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It sounds like you have a particular viewpoint - atheistic ethical relativism. I'm sure you can appreciate that the existence of god and the existence of universal ethics are two separate questions. (Example: a universe with a god, just one that is morally ambiguous.)

Of course Taleb has condensed several different thoughts into a single aphorism. One might be that laws (civilizations, governments) tend to not last as long as broad ethical traditions. Another is the observation that what is legal is rarely what is right. Yet another is framing what should be a guide to your behavior: do what is most consistent with what you believe is right, not what is legal.

Some of what you are saying is related to the grand challenge of ethical philosophy. But your solution, relying on the law, will probably result in some pretty bad consequences, especially when you think about how laws are made - and that laws are sometimes used as a cover for bad behavior. Taleb is saying (among other things) that you should think about your ethical choices on the terms of your own ethics rather than appealing to some other authority (the law) as a guide for your actions. He is being a bit pretentious while making this point but the logic is sound, I think.

Personally I agree that Taleb's point about grandmotherly knowledge is a bit silly, but I think the underlying idea is well taken. Absent any fundamental understanding of how the world works, people still had to figure out how to survive in that world. Those that survived probably have made some choices that - perhaps in hindsight - aren't that dumb. That being said, the world is changing faster than ever before, so I think Taleb's analogy is very weak. I also find his point to be in contradiction with other things he has said before: notice that he has assigned some meaningless quantification of uncertainty ("90%") to the quality of this grandmotherly knowledge. Such a number has no basis in reality, and stinks of the type of probabilistic ignorance he rails against.