Having reread it, I think its a measure of the difference between [what people are saving now] and [what they should save if they are going to live to 100]. Part of the gap is that people aren't saving enough for retirement with any longevity.
It could also be the difference between how much [the average person needs to save with a sustainable state pension based on current longevity] and [the average person needs to save with a sustainable state pension based on average longevity of 100].
Also, if the editor decides that they want a spectacular story, "safe withdrawal rate means longevity is broadly irrelevant" doesn't really cut it.