Twin and War Caster have always been different enough to have unique rulings. Twinned Spell requires the spell to be incapable of targeting more than one creature. For War Caster, it doesn't matter how many creatures the spell CAN target, only that it IS only targeting ONE.
— DMDanDanFielding (@DMdandanfieldng) November 11, 2020
Twinned Spell and War Caster in D&D both care about number of targets. But their descriptions define that concern in different ways, and they intentionally play differently. @DMdandanfieldng describes perfectly below how the difference in wording creates different effects. #DnD https://t.co/HWX3rfdnAs
— Jeremy Crawford (@JeremyECrawford) November 11, 2020
For an easy example, twin doesn’t work for magic missles but war caster does provided they have a single target