You appear to be the one applying the uniformitarianism logic not MrB.
Furthermore, please explain how you get xenoliths like those found at Mt. St. Helens where everything should be dated the same, but some blind samples were coming back from the lab at over 1 million years old iirc.
If it laid there in a solid state, decaying for one million years, and the got ejected from the volcano it's still going to show a million years worth of decay. If you melt it down and let it cool, the same ratios of elements are still there. Assuming that they should all be dated the same starts with an assumption that every bit of ejecta from that eruption is exactly the same at a molecular level. I'm pretty sure volcanic eruptions don't follow that pattern.