You are confusing the equivalence of the result of the operations with the equivalence of the operations. The operations are not equivalent. That is the point. The question is not testing just whether you arrive at the correct result but whether you arrive at the correct result via the correct operation. The question clearly prompted the student to apply a particular strategy, and the student did not correctly apply that strategy. It's very cut and dry.
There is also a big difference between teaching a complex subject to a student and laying the groundwork for a later class. You can see that in the array problem on the same test. That's introducing matrices without getting into full pre-calculus yet.
