You mean I don't think this is automatically correct. Otherwise it very likely is correct. Either way, you're guessing the manual curation is done in a way that is favorable to include I don't know answers. Which it most likely doesn't.
I don't know why you assume it's a guess. These providers employ thousands of people directly or via a number of intermediaries to work on their SFT datasets.
To train this effectively you would need a dataset of questions which you know the model doesn't know. But if you have that... why not answer the question and put in the dataset so that the model will know ?
That's a bit imprecise, but I think it capture the idea of why 'I don't know' answers are harder to train.