The original post only gave half the explanation. It's not that lead exists in general, it's that lead exists within zircon crystals.

Under normal circumstances that would be impossible, zircon crystals strongly reject lead atoms as they form. There's no way to stuff lead into the crystal lattice in the quantity we find them there. But uranium and zircon go together just fine, we just have to wait for it to decay into lead. The trouble is it takes ~4.5 billion years for just half of those uranium atoms to turn into lead. So any zircon crystal we find with half as much lead as uranium must be roughly that old