I think you misunderstood.
I did exactly this (default is all new icons to a particular Fence) a while ago because I don't want all icons in a Fence.
So new icons DO go into this Fence.
But I also have a lot of icons that either:
* aren't new or
* I purposely put on the desktop outside of a Fence. (Dragged out of a Fence to the desktop.)
What I'm telling you is that... in spite of this...
Sometimes, icons sat on top of each other on the desktop NOT in any fence (assuming that Fences created this situation)
Until I rebuilt the Portal Fence today, icons were sometimes sitting on top of each other within that Fence.
In any event, right now, I'm okay.