Yes, I saw those posts. I do mean it opens a new window...
I have "open_files_in_new_window": false,
keep in mind that open_files_in_new_window is marked "OSX Only" - I'm not using OSX, I'm using debian.
Still scratching my head.
Additionally, the same thing happens if I open the project from Gnome 2's file folders. I can, however, drag the document into the SLT window and it will properly open a new tab.