This article seems targeted at those who are young, and plan to retire at a "normal," retirement age. The point it seems to be making is that the normal retirement age will increase, unsurprisingly, and that those who plan on working until normal retirement age need to adjust their expectations. People are living slightly longer, in terms of absolute age, rather than an increase in the number of those surviving long enough to become elderly. The maximum age has been increasing for about 1 year every decade.
There also has been a very significant increase in those surviving to a normal retirement age, this is also alluded to in the article, though not discussed much. Most of the increase in longevity comes from an increase in the survival rate, so most of the increase in the older workforce more has to do with people having a longer healthy period, rather than their maximum possible age increasing.
But for those seeking early retirement, this article will be mostly irrelevant, it just illustrates that people will have to be on the hamster wheel longer to hit the point where companies start kicking them off the wheel (and probably getting retirement benefits), than their older peers. E.g. For those employers who still somehow keep around a defined benefit plan, it will take 35 years to vest, instead of 30.
Whether it becomes harder to retire early as someone young, is not clear. It will certainly become harder to retire early in industries where there is no growth, and as such, a larger pool of healthy workers will be available, but might not be the case in growing industries. Since early retirement requires a much shorter work-cycle, however, workers on such a plan will be substantially less affected by the increase in the number of healthy workers, and the inevitable changes in growth that can occur in particular industries (job risk in other words, less time is spent in the "market").