The occurrence of radiogenic daughter isotopes such as lead-206 in the Earth's crust is a matter of established geological fact resulting from the radioactive decay of uranium and thorium series.
The claim states that Lead 206 and other daughter isotopes occur naturally in the Earth's crust. This is a foundational geological fact taught in introductory science (radiometric decay of uranium/thorium series), making it common knowledge. Several papers reference crustal lead isotopes and natural radionuclides (U, Th) in rocks and sediments, supporting the natural presence of these isotopes.