>but if you follow the curves straight down, you see that the cutoff is hovering over or near the 90 S line.

From this single view I could see it in this way, however I could see it still in the other way, but I now found also this image, which shows that most methane emissions are coming from emissions on land and so your explanation of the 3D projection makes sense if the 3D projection displays methane concentrations near the surface. (....while not really knowing what is meant with computer model...does this mean its no measurement?!)
This image displays also that in the stratossphere the situation looks quite different. So it seems circulation patterns are also different in the stratosphere.

and it seems also that in particular not yet so much of the antarctic methane has appeared, at least I haven't seen reports like as they exist for the arctic and it doesnt show up in the 3D projection or the computer model image.