I'm glad someone finally stated that. AAC is not just any standard, it is the MPEG/ISO standard.
IOW, AAC is to MPEG4 what MP3 is to MPEG2. As you stated, AAC is the official successor to MP3. That's why Apple chose it when they did. At the same time MPEG4 became their standard for video AAC became their standard for audio. That was before the iTMS even went on line.