If you have any reasonable understanding of the principles of flight, then you come to realise that all of the exam syllabi are gross and often innacurate simplifications.
However, the objective in the short term is to pass the exams - and it is true that if you replaced the wing section with just the upper surface you'd lose about 1/3 of the lift. So, the answer works - but is still grossly simplistic and pointless.
Learn to pass the exam, then get a decent book on PofF like Kermode or Anderson, and learn it properly!
Daft, but the way to do it.
G