The reason I'm asking this is exactly their blog post, which says says that their goal is to roll this out to 50% of the French queries at the end of the year.
That's a rather limited slice of traffic (50% of French queries, 0% of everything else), and it's not described as the current state but their goal for the end of the year. The obvious implication is that right now even their French rollout is lower than 50%. How much lower, we don't know for certain, but 5 months is a long time. I have to imagine it is far lower than 50% right now. 1%? 10%? Hard to believe it is any higher than that.
So I think my original question was very fair. The answer could for example have been that they indicated on the results page, or maybe they had some kind of a per-user opt-in toggle that you switched. From your reply, I gather that's not the case, and you're just assuming that your results must be from the new index. That seems like a bad assumption to make.