You can't talk about modularity without touching on shared mutable state. Shared mutable state is the fundamental primitive that eliminates modularity. You get rid of this, you're entire program is now modular.

None of the writing really gets deep into this so I assume the author doesn't know.

It's not "mansplaining" you social justice warrior. I don't even know the sex of the author and I don't care. Don't turn this into some sex based conflict. It's called explaining, and that's all it is.

I'm assuming you don't know about it either so I suggest you read my "explanation" as well.