Can confirm - tests are wrong: one glaring example: 123 = 1+2+3 = 6. Test expects that to be 9. Hackerrank is enterprise software confirmed.

For anyone struggling with the last test case and getting overflow: you might have to use a 64-bit integer to hold the super digit value, even if you choose to return a 32-bit integer at the end:

int superDigit(string n, int k) {
    if(n.size() == 1 && k == 1) return stoi(n);
    long super_digit = 0;
    for(char c : n){
        int current = c - '0';
        super_digit += current;
    }
    super_digit *= k;
    return superDigit(to_string(super_digit), 1);
}

setting the n constraint to 10^100000 is pure evil lol

Not correct all tests

Tests are not even correct.