A possible scenario is awarding ACs to all the users who pass even a single subtask, as that wouldn't take away any accepted solution of a participant during the contest.
There are much better scenarios available. I understand that you dont want that someone who got AC during contest gets TLE and WA now, but we also need to look at the cost. Short contest is more of time and accuracy. If a user submitting a brute force solution is getting ahead of users submitting correct solution, then its really wrong.
That Q has 1262 correct submissions, and thats ~66% of entire population. Ranks 175-1262 were determined by this problem. A huge margin. If someone genuine gets his ratings reduced because other people’s brute force got accepted, then this wont be fair to him either. Infact its a lot unfairer than giving WA to brute forcers after contest.
Not only that, this system of AC on passing 1 subtask might result in other complications too. Someone getting WA in higher subtask but AC in lower one, or vice versa, and deserving WA has got AC.
The contest will remain rated.
Whatever you do, just remember, its better for contest be unrated than unfair. Its tough to get a “fair-to-all” solution, especially when population involved is large percentage wise.