Hi again,
So I talked with the team and passed them the documentation for the Optimize API. The API provided by Optimize seems to be a way to display a given variant (A/B) to your user's website, not for an application like us to create experiments programmatically. Although we'll have to look further into it.
So for the moment, the best way to do A/B testing with Prismic is still something like Optimizely I think. You can also try Renaud's recommendation above.
Thanks.