Zooming into the image, by “all one piece,” they mean the grates and bottom plate, as there are seams along the back wall at about one average bed width apart

Ah, noted.

I had this happen on my ender 3 v2 recently as well, a thin layer of washable purple gluestick has solved it for me (though parts are now sometimes a bit difficult to remove and sometimes require a moist rag to get the gluestick residue off)

In my experience this is just how 90° overhangs are. It can’t be perfectly flat because if it was it would fuse to the temporary supports. You might be able to get better results with better fillament cooling (or lower fillament tempatures), but afaik some artifact will always be there. So unless I’m mistaken you can’t really “fix” this issue and you are most likely better off designing around it and adding some extra tolerance when you have to use overhangs like this.