A opened polysrf is identified as a closed polysrf

I am trying to extract the intersecting object between a cube and a swept solid by using booleansplit, but it ended up with an opened polysrf that is identified as a closed polysrf by rhino. It doesnt allow me to create a closed mesh which i need to export as STL file for 3d Printing.

Here is the file

Booleansplit.3dm (8.3 MB)

Hello - your input object is full of self-intersectiions - that would need to be sorted out first, before the Booelan operations.