right but OP’s solution for this part of the problem is assumed to be instantaneous (ie O(0))
Although: that's not true for arbitrarily sized integers either. Multiplication in O(1) implies P = NP (which further implies NP = PSPACE): https://cs.stackexchange.com/a/1661/129151
In terms of algorithms, I suspect that no actual algorithm fits into it and as such it is more a curiosity than actually useful.
However, O(0) is not the same as O(1), as we cannot find a constant c such that 0c dominates all possible functions that 1c can dominate.