I think they aren’t saying that it’s a hard profession by itself in the way that you are talking about — only that the expected returns are negative for almost anyone.
So in that case it’s not calling day traders smarter or harder working than others. It’s saying that no matter what your level of intelligence or other personal attributes there is likely some other job that would have much higher expected returns.
For person A it might be to be a firefighter, for person B, to be a gymnast, for person C, to be a brain surgeon, for person D to work as a maid. Etc etc.
At least that’s the way that I read it.