Your reasoning is completely right, without changing the cooking speed the business is not profitable. If you read carefully that part, the author says:
Assuming that the employee cannot work faster, this loss will get bigger the more our sales grow.
And then he suggests a potential solution that is
Barb could find an employee that works faster
But how much faster? To answer this question he calculates that an employer should be able to produce 9 burgers/h in order to become profitable.
Is it more clear now?
Feel free to contact me if you want to discuss more about it.