Proportional to the degree of damage that can be done by the employee in question... yes, absolutely. If you have the responsibility and authority to disburse millions of dollars to a random bank account number, then you've got a high degree of responsibility not to be spear-phished, and it would be a disqualification if you are unable to resist it.
On the other hand, firing a front-line call center employee because they failed the spear-phishing tests is fairly pointless and more damaging than helpful.
Where exactly the line falls would be up to the business and like so many things, involves too many factors to be reasonable to discuss here. With the typical concentrations of power and authority in a business, it's only going to be the minority of employees that would be faced with termination for this problem, because only a minority will have the power to do significant damage to the business in general.
I think it's not too difficult to think that the article is mostly talking about the situations where it isn't proportional to the degree of damage that can be done by the employee.