Clearly the kid doesn’t have a deep conceptual understanding of how to solve equations or simplify expressions. True, the kid probably learned some stuff proceduraly as opposed to conceptually. (Though, I can confirm, that in this classroom nobody ever said anything, like, “When you have an equation you need to add something to each side to isolate the x.” The balanced-scale model was used at first.)
There’s still two interesting, deeper questions, to consider. (Possibly more: bring it up in the comments.)
a) Would this kiddo always make this mistake, when presented with an expression to simplify?
b) If not, then what exactly is it about this problem that prompts the kid to employ a basic move from equations?