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@ecsd It says that. The big decay chains terminate in lead 206, 207 and 208 and primordial lead is lead 204 (with some minor decay chains terminating there too).

It's quite astonishing! I had your reaction when I heard this argument and I looked it up.

It actually makes sense if you imagine that the primordial distribution is pretty even between lead and the next heavier element etc. After a few billion years, radiogenic lead, the decay-product lead, represents a large chunk of all that mass that was initially heavier elements. Even if there were only two elements heavier than lead, that would already mean most lead today would be radiogenic.