First of all, you don't have to assume anything. As soon as you get the privilege of reviewing posts in a specific queue, you can switch to the history tab to see which actions have been taken by whom. For first posts, the history can be found here.
In this history, if you click on the action that has been taken, you're lead to the review task for that specific post. That information is completely public, as long as you have a direct link. For the post from the screenshot, you can find its review here.
As you can see, only one user (you) has reviewed the post, which completed the review task and thus removed the post from the review queue.
While it is possible that there is a bug that doesn't record all reviews, I think it's far more likely that the commenters either forgot to click I'm done (still unlikely) or that they simply found the answers while browsing PPCG and didn't use the review system at all.