Indeed, I wouldn't expect to know about this wrinkle and I wouldn't expect an average person to know about it.
But I would expect a (supposed) expert in nuclear power who writes an article titled "why I am not worried" to know about this.
And while I am not an expert, I do know enough to know spent fuel has to be considered part of the system of a nuclear power plant and thus keeping it safe is part of keeping the entire plant safe.