I am not sure if this is fully there, but I feel something like a double exclusion might be the way to achieve this....
However, this would require precise testing in an isolated environment where you control all the visits and sections on those visits to ensure this is working as intended.. I do not currently have such data to fully test with...
But I feel that Something like:
Visit
Visit Container (Exclude)
Dimension contains "/site-section-1"
AND
Visit Container (Exclude)
Dimension does not contain "/site-section-1"
Essentially, I am excluding Visits that contain "/site-section-1" but then from those visits, excluding visits that that have anything that is not "/site-section-1" (basically a double negative... so the logic is hard to explain)
I did a quick test with three columns side by side... basically unfiltered, next to where this was applied, then a third column where I did a simple "Visits contain X".
My unfiltered column obviously had the most data, then where this was applied had less.. then the third column where I stacked with must contain X (the item we are trying to block when the only section in use), and the metrics were even smaller.
This says to me that my initial segment should be allowing visits that don't hit X at all, and also allowing X where there are other sections.
But if you build this, you really need to fully test it under controlled conditions... it might require additional tweaks....
On the surface it looks "okay" but it's hard to tell.