I think the writer had multiple relatively weird confusions, to be fair. It's most likely that "a little knowledge is a dangerous thing". They obviously knew something about escape analysis and Go's ability to put variables on the stack, and they likely knew as well that Go slices are essentially (fat) pointers to arrays.

As the author shows in their explanations, they thought that the backing array for the slice gets allocated on the stack, but then the slice (which contains/represents a pointer to the stack-allocated array) gets returned. This is a somewhat weird set of assumptions to make (especially give that the actual array is allocated in a different function that we don't get to see, ReadFromFile, but apparently this is how the author thought through the code.