Wouldn't it return the size of the pointer? I would guess it's exclusively used to handle architecture differences
Nitpicking aside, the result of sizeof(*src) would be the size of the object at which the pointer points. The type of that result is size_t. That's what makes this code from the lesson I quoted invalid:
*sizeof(*src)
That first asterisk tries to dereference the result of sizeof as though it were a pointer, but it's a size_t: an unsigned integer type. Not a pointer.