Isn’t AAC an Apple-specific format that isn’t portable to other devices?
Although Apple has embraced the AAC format, it didn’t invent it. The only Apple-specific part of old iTunes files is that they’re wrapped in DRM (these music files end in the file extension .m4p—the p stands for “protected”—as opposed to .m4a). Apple’s DRM is only compatible with Apple products, but AAC files play on a wide variety of players.