So it would detect you for example as some sort of millennial, and use the punctuation that was passed down to you as correct and a signal of competency

Yes, we think of this as a decision tree, where initially, copies may look different by say, user demographics and location. As we learn more about what's working well for different dimensions of users (eg: topical interests, traffic source, platform type), the decision tree grows, and every single element in the anatomy of copies is optimized based on past learnings. In an ideal world, every user truly sees a unique copy tailored to them.

How do you do that in a meaningful statistically significant way?

Not a gotcha, just genuinely curious.