From the feat description:
You learn the misty step spell and can cast it once without expending a spell slot. [...] Intelligence is your spellcasting ability for this spell.
As far as I've been able to determine, spellcasting ability is only relevant when there's an attack roll involved, there's a saving throw, or the ability modifier augments damage. In the case of Misty Step, none of the three apply. There are similar cases with other magic racial feats ("Charisma is your spellcasting ability for all three spells," where one of the spells is Detect Magic).
So what's the significance of naming a spellcasting ability?