It's possible that the human baselines were bored and so performed sub-optimally when picking between the 1K classes. But the argument has now become a subtler one, much less clear cut.
As an example of categories that may be difficult to distinguish between, do you feel confident that you can reliably distinguish between the Norwich terrier [4] and the Norfolk terrier [5]? These are two separate categories in ImageNet1k.
[1] https://i0.wp.com/blog.piekniewski.info/wp-content/uploads/2...
The first diagram shows exponential growth in the compute usage of state of the art deep learning architectures.
[2] https://i1.wp.com/blog.piekniewski.info/wp-content/uploads/2...
The second diagram shows diminishing returns on Imagenet1k top-1 accuracy from doubling the size of Resnext.
[3] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.725...
[4] https://www.google.com/search?tbm=isch&as_q=norwich+terrier&...
[5] https://www.google.com/search?tbm=isch&as_q=norfolk+terrier&...