The original tweet places the blame on the wrong person. As a manager or boss, if you put your company in this position in the first place you bear responsibility for the risk. If you discover an employee has this kind of leverage you have not done your job.
Many managers don't consider this kind of risk until they have to react to losing a key employee. Many more managers don't know enough about what their direct reports do, or how the organization works. They have never felt the responsibility or had the leverage that a key employee does.
I once worked as a contractor for a company that ended up with me running all of their IT infrastructure and maintaining their software (SAAS company, so their only product). I repeatedly told them they should hire a f/t person so they didn't depend solely on me, but they didn't want to spend the money or do the work, for years. One day the owner reviewed my retainer and asked me "What exactly do we pay you for?" I told him "You pay me not to quit." He didn't like hearing that but even then didn't do anything about the problem.