Despite the fact that there are many scientific problems with radiometric dating, there is a more significant problem.
The claimed “fact” that decay rates have always been constant is actually an inference based on a uniformitarian assumption.
It is true that radioisotope decay rates are stable today and are not largely affected by external conditions like change in temperature and pressure, but that does not mean that the rate has always been constant.
There is also a difference in the timescale used to explain the layers.
Determining the relative age of a rock layer is based on the assumption that you know the ages of the rocks surrounding it.
If certain things are known, it is possible to calculate the amount of time since the parent isotope began to decay.