Two things:

• A solution of def nthFibo(n: Int, a: Int = 0, b: Int = 1) exposes the implementation detail of a and b in the interface.
• The reference solution ref is referencing the submitted method nthFibo.

Interesting: The Haskell version using arbitrary precision integers for the result. I tried the same with the Scala/JVM Integer and the solution could not complete in time.