We've run into an issue when a person with admin rights is taking a New Quiz. Even though this person is enrolled as a student in the course, due to the Admin privileges, that person cannot complete the New Quiz. It only shows the "Build" screen as opposed to giving them the option to submit it as a student.
The only way around it we've found is for the Admin to use a demo account as a student to be able to submit the quiz.
This functionality will severely limit our ability to use New Quizzes in faculty supporting courses moving forward unless there can be an update to allow faculty/admin rights to be able to submit the quiz as a student.
Is the the user an admin on the highest level of your Canvas account?
If yes, then you are correct the only way to take a course, is to have a separate student account specifically for that course.
If no, the user can be an admin on a subaccount and take a course in another subaccount as a student.
For example the main account is called Acme College, with subaccounts for each department such as English and Math. If the user is an admin on the Acme College account, their admin rights will apply to all subaccounts and therefore all courses will be viewed as an admin. If the user is an admin on only the English subaccount. They can be enrolled as a student in a course that lives in the Math subaccount, and see everything as a student.
This is by design. Depending on how you've configured your permission settings, an admin user is going to have permissions to view or edit a quiz in Canvas. Even if they are enrolled as a student in the course, 'admin' has higher permissions and would supersede the 'student' permissions. You would need to house your faculty training in a subaccount where these users do not have admin permissions.