With parallel arrays in their pure form, the things in your arrays are always primitive data types like characters, floating-point numbers, or integers, and each array is homogeneous. With nested records, you have heterogeneous data collections (records) and maybe even arrays of them. So, yes, column-oriented vs. row-oriented — or potentially some more ramified structure of rows containing rows containing columns containing rows...

I agree that what kind of arithmetic your computer uses is mostly irrelevant to the memory model.

I'll see if I can clarify these things in the essay. Thank you!