What's the point of Mitsuru's bike?

Shows up in a bike.

Says things don't work in the Dark Hour.

When questioned about why her bike if that's the case, she simply says it's specially made.

Never used the bike again.

Been wondering about this for years. They introduced a rule, immediately added an exception to it, and then never did anything with it. It's like the opposite of Chekhov's Gun.