Well, as it turned out, I was mostly wrong: “atomic weight” refers to the average weight of the most common isotope mix of the element (on Earth), and our data about the isotope distribution changes. I didn’t remember that, but that’s probably what that original quote meant.
Although you are right, as a nitpicker I don’t think non sequitur, i.e. “doesn’t follow”, is correct to use here.
Well, as it turned out, I was mostly wrong: “atomic weight” refers to the average weight of the most common isotope mix of the element (on Earth), and our data about the isotope distribution changes. I didn’t remember that, but that’s probably what that original quote meant.
You seem to be right about the non sequitur.