Similarly in standard notation, it is perfectly fine for x to equal 7, 3+4, 10-3, 3.5*2, or any other combination, so long as it is 7. The fact that you assigned values to the opening and closing parenthesis doesn't change the fact that using the meaning of the equal sign, you came about the correct answer, whatever form it may be in.
Math is not a guessing game. The problem is that students only memorize procedures so when they encounter a problem they can recreate the solution. When you rephrase the equation into 4+3+2=x+2 (Solve for x), you are merely putting the question into recognizable procedure. This does not mean that he understands what he is doing, as is shown by the article.
Plus, we live in a world where variables are expressed in a variety of different ways before we even get to the great and awesome x. Square boxes, underscores, question marks are several that I remember going through before x. ( ) isn't that much of a stretch for a young student.