This could in theory happen, but in my examples I'm adding the url right after someone converted -- paid for a subscription or completed an order. Those are unlikely to be shared with someone else (ideally). It's arguably more likely that the user will share it with themselves on another device, in which case the overall approach will work well.
I should also point out that the url tracker isn't meant to be persisted across page views. It's only done once at the moment that the user identifies themselves to your service.
Author here - we've been able to identify anonymous users pretty consistently once they convert to becoming users. This talks about our approach and how to do it, while still following all the rules around tracking cookies, etc…
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).
We worked with them over the course of a few weeks. Given how complicated the topic was we came to them with copy, layout and a few loose ideas for what graphics we need. The design and graphic work was all them though.
If you have any other questions about it happy to dive in deeper!
I should also point out that the url tracker isn't meant to be persisted across page views. It's only done once at the moment that the user identifies themselves to your service.