Possible error in solution

Optimization 1 in the solution to Connected Components? states that the time complexity of unordered_set.count is O(1).
The documentation states that the time complexity is worst case n, and average case 1.
Is it okay to assume that with a large enough n, the running time will approach the average case?
Is the solution wrong?

yes it’s wrong; see (Optional) More on Unordered Sets & Maps