My memory is that it was common knowledge for several years that upgrading the OS to a version more than 3-4 years past the hardware release would cause a dramatic performance hit. For as long as I had access to an iphone, that was a known issue, as in, many people with older iphones would agree, any apple forum would have topics about it. Maybe it got more press coverage in the year leading up to the admission. Maybe there were multiple factors and the one they admitted became prominent in that year.