The Assembly of State Parties is comprised of States that have ratified the ICC.So, the SS might have been implying that since some of us haven't ratified ICC (USA, China) the SS cannot be the best place to be asking for a deferral.
Secondly, since AU says its members are the majority in the ICC, then they should use that avenue, Assembly of Parties to ask for deferral.
If you stay ready, no need to get ready.