I'm no English major but let me explain why it works for me. Firstly, there is a different tense between "was to remember" and "remembered". The former maintains the vantage point of now; we're not quite there, we're looking to the future still. Secondly, the passive voice of the former creates a juxtaposition of its more clinical style against the setting of a firing squad. Both of these choices serve to stop us from putting ourselves directly in the mind of Buendia. Strangely, counterintuitively, it strengthens the impact on us of the scenario when the full meaning finally lands, which the dispassionate narrative style has obfuscated.