Let us remember that it is not only the essay "Aria" that is not writtten in sequential order. The other five essays of this wonderful collection also feature a non-chronological approach to time as Rodriguez jumps around different parts of his life as and when it suits him. I suppose the answer to your question relates to the way in which Rodriguez himself called his work an "intellectual autobiography." The structure he adopts therefore for this work is focused explicitly on his intellectual development and his growth and maturity rather than on passing events and every section of his life. He deliberately therefore eschews a linear, sequential narrative and jumps around by focusing on the stages in his life that have had most impact on his intellectual development into the man that he is today.
Rodriguez never claimed to be writing a simple autobiography, and he is always clear that he had another agenda in mind when he was writing these six essays. The non-linear narrative can therefore be seen as a product of this other agenda and his description of his work as an "intellectual autobiography."