Hey Luke,
I didnt see it either. I think it is a "post boolean" item. Not pre boolean

If you zoom to where your green object is "Supposed" to touch with the other object in the front view, you can see the gap that would create a tiny sliver.

I zoomed in to make that gap large enough, then selected the green object and used the edit frame to drag its size to snap the other object, then I did the boolean.