I think he's hoping for both an attack and a handover of reclaimed Raqqa to Assad's government, rather than to another faction favored by Turkey, the US, or the Kurds.
So, he's jumping one step ahead more than hoping ISIS can hold Raqqa, as its fall is a foregone conclusion now.
Ja’afari is a Syrian diplomat speaking at an International Forum, he is just citing International Law, not endorsing ISIS or a city they hold as “legitimate”.