It's odd because I've read that before and made an assumption that it was talking about AAC mastering, but it only implies that. There's nothing that comes out and says what they want! There are some parts where they suggest higher fidelity sources, but mostly it comes down to:

    Apple Digital Masters Droplet 
    You can use the Apple Digital Masters Droplet to automate the creation of 256 kbps 

AAC encodes.

There isn't actually anything about what you're supposed to send them, only the recommendation to use their tooling which seems geared to produce 256k AAC files.