There's no such kind of requirement regarding the filter. In my reading it is a precondition.
"takes a list of non-negative integers"

You missed the "non-negative integers" requirement.

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It should be noted that the angles provided and expected are interior/internal angles, not exterior/external angles.

I think that requirement is basically a red herring. Accounting for case means the check should be case sensitive. So the assertion you gave should fail.

You really need to optimise your code to be able to pass this in C#. I wouldn't say it's tremendously tricky to do, but it does require some thought.

But I agree that, as far as C# is concerned, this is more difficult than 7 kyu.

Not quite. They're similar, but Triangular Treasure has the added challenge of input validation.

Although it may seem obvious, the description probably ought to specify that the answer should be in degrees, as opposed to radians or gradians.