Does server-side authentication in ReactJS or any frontend result in tight coupling with the backend?

Question

I am learning ReactJS and got started with authentication.

In one of the lectures, it was said that "server-side sessions are a great way of enabling authentication but they do require a tight coupling between backend and frontend."

Here is my understanding of server-side sessions -

1. A user logs in through a form on the React JS front end.
2. The credentials are sent to the backend server.
3. Upon successful authentication, the backend generates a session token (which is stored on the server, mapped to the user ID or some other user credential) and sends it back to the front end.
4. The front end includes this token in subsequent requests, allowing the back end to identify and authenticate the user.

Here is my understanding of tight coupling - Changes made to one component result in making changes to the other component. Here, components are dependent on one another and are less flexible.

I did not quite understand what they truly meant when they said "server-side auth results in tight coupling" and how said tight coupling can exist.

Is there a scenario out there where server-side sessions do create a tight coupling?

If so, then why is it a popular authentication mechanism and opted by many?