Yeah, this post is not fully correct. The lead nail in the coffin is not that lead exists, it’s that we find it in certain mineral matrixes that don’t form with lead.

Zircon is the most widely referenced mineral in uranium-lead dating, as the mineral rejects lead during its formation, but will incorporate uranium. So when we find zircon with lead in it, it means that the uranium has decayed and turned into lead while being stuck there, and the percentage of uranium to lead in a sample lets us determine its time of formation.