One possible hypothesis could be: since diabetes causes accelerated accumulation of AGEs, and Alzeihmer's is correlated with accelerated accumulation of AGEs, and processed food is high in AGEs, consumption of processed food is also bad.

One way to negate this hypothesis would be to show that the accumulation of exogenous AGEs is insignificant compared to the endogenous production of AGEs. I'm not knowledgeable enough on the subject to say if this has been studied.