Autocracy for Culture Victory is entirely viable, and its all down to the stacking % bonuses to tourism for multiple shared wars. It certainly takes more thought and timing than a Freedom-based Culture Victory, but its potentially a lot faster.
So, optimal? Depends what you mean by optimal.
Is it the fastest way to Culture Victory? No, because you don't need ideologies for the fastest culture victories.
Is it the fastest Ideology-based way to Culture Victories? It has the
potential for the fastest culture victories, but is less consistent in speed than Freedom and harder to execute.
Is it the most optimal Ideology for Culture Victories? Only in the sense that it can generate the highest tourism per turn.
Returning to the opening post the answer is as follows:
Since autocracy has all these bonuses for military and is a lot easier for autocracies to make military, is it necessary for autocracies to achieve a domination victory? Can autocracies also achieve a science victory? Same question with the other ideologies that have come out and have the missing trait. (I.e. warmongering freedom)
You don't have to do anything with any ideology.
You can achieve any victory type with any ideology.
It is, however, a good idea to use your military actively if you're playing Autocracy.
Using your military doesn't necessarily mean you have to go for a Domination Victory. Military action can play a strong part in all victory types.