That is indeed an interesting question which is probably relevant for quite a lot of users, so I'm happy to give you my perspective on that:
All coachees of mine who went through the McKinsey recruiting process gave me the feedback (which is also my understanding) that the McKinsey Imbellus is not a decision criterion on its own and thus you don't get separate feedback on just the McKinsey Imbellus.
Instead, together with the one first round interview you will be conducting, you will be assessed based on your performance in both formats. The feedback provided will focus mostly on the first round interview as the second round interviews are of the same format, but you can nonetheless ask about your McKinsey Imbellus performance as well.
The key question from my side would be if you have already received an invite for this one first round interview. If not, I would advise you to contact the recruiter to do so.
In case you want a more detailed discussion on the next steps in your McKinsey recruiting process, please feel free to to contact me directly.
I hope this helps,