The DXF file has 75 polylines with the "Visible" flag set to false. When we convert them to Linestrings, we don't retain the visible flag.
To see the polylines, select all objects (Ctrl-A) and then hold the ctrl key down while selecting everything in the view (window select).
You can then set the visible property to true using the properties pane.
The select "all" command will select objects that don't display (so useful to find zero length lines). When you unselect the visible lines (using the ctrl window method), then just the polylines with the visible flag set to false are left.
Another way to select the polylines is to use "advanced select". Use the "Data with the following property" method.