This is one of the instances when the FARs are pretty black and white. No interpretation is needed. If someone else is in the airplane, you are not solo.
What is there's a dog in the back? Are animals considered occupants? What about infants? What if your grandma is in the back, and she's in a coma? What if grandma's corpse is in the back? Is a dead person considered an "occupant"?
The point I'm trying to make, is that there are very few "black and white" regulations. Just about everything can be argued one way or the other, especially this one. You always have to ask yourself, "what is the intent of this regulation". Sometimes the wording doesn't match the apparent intent, and some questioning is required.
In this case, what do you think the intent of this regulation is? Would your grandma in the back in a coma violate the intent of this rule? Would a 3-year old? Would a 10-year old? Your wife? A student pilot? An instrument student? A CFI-applicant?... Were is the cutoff? Unless you answered "no" to each one of these, it's not "black and white", as you put it.