If a problem is “not semi-decidable” and “not decidable” can we say it is “undecidable”?

Question

I was under impression that when a Language (or problem) is not semi-decidable and not decidable then we can say it's undecidable and I think it makes sense also based on diagram.

However, in my assignment the TA struck out "undecidable" and wrote "not semi-decidable". Why is "undecidable" wrong if it's both not semi-decidable and not decidable?