Again, as an adult looking to find something to be outraged at, you are far overthinking the problem. You assume those kids don't understand what that week's math lessons were about. And therefore what any quiz/test would be about at the end of the week. All of them would have been coached all week long on what to look for in that quiz/test.
If the teacher was so wrong, explain to me how a majority of the students would have understood that question and been able to figure out the correct answer and provided the correct format? Getting one odd answer on one test/quiz in a room of perhaps 20 students is not indicative of a poorly written question or if a teacher is unwilling to admit they were wrong. Odd answers are just generally an isolated issue, unless this is a repeated problem for this student, which would be indicative of a deeper learning issues. Which is something we don't know or can't know in this case.
Your premise would hold value if you knew every student in the classroom got the question wrong or provided the same answer that is shown. But you have no idea if that's the case.
There are other things in this world that deserve to be outraged about. This particular thing ain't one of them.
Sometimes you have a run in with a customer that ain't worth having-- no matter how much money they pay.