An ultimatum would apply where there is no legal basis such as between countries. An ultimatum contains a deadline. 'Immediate' is a poor substitute for a deadline, so this ultimatum is deficient and is not an ultimatum at all. The attack might as well begin without further ado.
I suppose it depends on how willing one is to enforce the word 'immediate'.....my defintion of immediate means 'now', this very point in time.....