It's been a long day, so I'm probably missing something; can somebody explain to me how "These shortfalls do not count, for example, those retiring baby boomer

It's been a long day, so I'm probably missing something; can somebody explain to me how "These shortfalls do not count, for example, those retiring baby boomers who would have left the labor force whether or not the Great Recession happened." I don't see it, since I don't see anywhere in their methodology that they took into account the changing age dynamic.