So technically it is an apology but in the other sense of the word :)

A defense is not the most common definition of apology in modern usage, but it is the most historical meaning of the word, and most aligned with its Greek etymology. This letter is very much a hybrid of both a defense and a mea culpa, the two modern usages of apology. He acknowledges some shortcomings which were partially responsible for him being criticized, but also explicitly rejects the accusations that he believes were unfounded.