HC
So faced with a suspected main rotor failure or very serious issue, it is perhaps not too surprising that the crew didn't think of punching out the AP as one would prefer to have as much going for one as possible when dealing with an emergency, as opposed to making life more difficult by killing the AP. In this case it was all over fairly quickly due to their proximity to the rig but had it happened in open ocean, who know what they might have done. It is very easy to pass judgement in hindsight and from the comfort of the armchair, somewhat harder to get it right with a massive flood of adrenalin and near-panic!
According to information, the same autopilot issue was evident outbound but the crew landed on the rig without a problem and attempted to troubleshoot (hard to troubleshoot an AP issue with all that metal around and while stationary on a rig) Then after departure when the issue came up again decided to ditch.
One doesn't need a comfortable armchair to suggest what may have been done for an improved outcome.