Good question, the idea in the post is once you know who the user is you make sure they load a page with a unique identifier on it that you can use to identify them.

As an example, think of a Shopify check out flow. Every user has a unique checkout url. Once they purchase you can use that checkout ID in your warehouse to join with the page view that had the anonymous Id on it. So you’ll have a page view with the anonymous Id with a url with a unique checkout Id that you can use to join to the ultimate identified user (assuming all your page view and Shopify data are in one place, your data warehouse).

Let me know if I understood your question!