Submitted Onenote assignments are not locked to editing when viewed in Teams
Once a student submits a Onenote assignment, the page should be locked to editing in the student notebook. When a submitted page is opened by the student with the browser or desktop app (Windows), the page is, in fact, locked and cannot be edited. If, however, the student views the assignment in Teams and then notebook page is opened in Teams, it will display in read-only mode. Then if the student clicks the "Edit Notebook" button the notebook page can be opened and edited in both the browser and the desktop app. It is effectively not locked to the student, even though the teacher view of the classroom notebook shows the page is marked as locked to students.
Teams Desktop and Browser