Daughter / decay elements of plutonium don't indicate the initial quantity, and might (I'm not sure of this) also be the result of other decay chains.

Radiodating used to determine the age of the Earth relies on uranium-lead dating using samples found within zircon crystals. The crystals cannot form in the presence of lead, so any lead found within a crystal must have appeared there after the crystal formed. As both the mother uranium and daughter lead are within the same sample, the ratio can be clearly determined.

https://en.wikipedia.org/wiki/Uranium%E2%80%93lead_dating

Absent a similar containment for plutonium in the formations being described in TFA, that may not be possible.

For C-14 dating, the ratio of carbon isotropes within living tissue is known. The residual C-14 is what's left after the original organism has died, and the ratio then determines age. Given the short timespans afforded by C-14 dating, concerns with contamination or loss are lower.