That's not the gotcha OOP seems to think it is. If the world was magicked into existence by a supreme being 4000 years ago, there's no reason it couldn't have been magicked into existence with heavy elements having decayed by an arbitrary amount or with Pb by itself. 'Tis the problem with invoking appeals to magic. And anyway a quick look on wiki says that primordial Pb was mostly created by neutron capture of lighter elements, not radioactove decay of heavier ones, so the mere existence of Pb proves nothing wrt the timeline of U decay anyway... but at that point if you're bringing nucleosynthesis into it, you may as well point to anything higher than lithium or even atoms as a concept as "proof" rather than picking anything as exotic as uranium decay.