What it teaches
Recall rather than working out. A duel is won by the child who already knows that 7 × 8 is 56, not by the one who can get there in fifteen seconds, and that is a different skill from everything else on this site.
The four wrong answers are the mistakes children really make: one out, ten out, the digits the other way round, and the answer to the other operation. A race against four random numbers would be a test of eyesight.
What to look for
Nothing from this game is recorded. The other player is usually an older sibling or a parent, and counting their answers as the child's progress would spoil the one number in the Parents Area that is meant to mean something.
Play it yourself against the child and pick a level that makes it close. A duel that is not close is over in a minute and never asked for again.