As this is an 8 kyu level Kata, I understand the intention of not needing to validate inputs as a way to keep it simple for beginners, but didn't get the point of including the function inside a class. Wouldn't that probably confuse beginners as they could think the class was a necessary element for the function to work?
Otherwise, good Kata for comprehending the usefulness of programming for little things like finding the perimeter of a rectangle.

This was my exact solution to this kata, but it was failing on the large array test due to timeout, so I had to change to a different more efficient solution.
Did the tests change in the meantime or was it a performance issue with the server? (<- rhetorical question)