It’s clear he’s observing something, but all too easy for the viewer to think the monkey is merely observing the pattern on the screen at the end. It also doesn’t help that there are a bunch of slight variations on the double slit experiment, like those listed in this post.

If the monkey was at 90 degrees to the experiment, looking at the slits only side on, then it might be clearer - though I’m not sure how you could draw that lol.