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?
Best Answer
At the moment it is rarely relevant...
No standard situations require that a creature utilize their spellcasting ability when casting misty step.
... but it could be in the future
If a later feature (such as a trap or a monster's trait) requires that a creature make an ability check using their spellcasting modifier for any reason, then this will be important. Say, for example, a monster that adjusts spellcasting making a caster roll an ability check with their spellcasting ability or the monster gets to choose the targets for the spell (with some DC). It is the general rule in 5e that any spell cast has a spellcasting ability.
P.S. I use this example, because I have created homebrew monsters with traits like this (among others) in my games.
Spell Thief
For an example of a feature that already uses this type of language, take a look at the Arcane Trickster's Spell Thief feature:
While misty step can't target an enemy rogue, other spells that typically don't directly utilize a spellcasting ability (such as magic missile) could. In these cases, the spellcasing ability would be used to determine whether the spell is negated and stolen by the rogue.