I am not in puter team, so I may not know some details.
But.
I don't really care how sign-in is implemented. If it is a popup, but simple for the user - that is ok for me. Probably they will change how it works in the feature because they Puter team was listening for my feedback with puter.js SDK.
Right now, I don't pay Puter. As I understand their long-term plan, eventually, they will monetize the users directly, for example, users will pay for bigger cloud storage.
For now, I don't have access to users. But I already spoke with Puter team about, and they told me they will have a full user management dashboard.