Twig may best be thought of as a more limited version of Tinderbox. Both apps share the same file format so a Tinderbox document can be opened in Twig even if some Tinderbox-created aspects can't be edited/controlled from Twig. Opening a TBX in Twig will not destroy non-Twig data. however, do not open the same document in both apps at the same time (though otherwise both apps can be open at once).
Twig documents (.twig) and Tinderbox documents (.tbx) are interchangeable though Finder will default the app association it creates to use whichever app first created the document.
The only non-Tinderbox feature in Twig is the Noter pane which brings together the Tinderbox text window, with the Find, Similar Notes and Common Words dialogs.
Twig lacks text/HTML export, agents (or creation/edit thereof), custom colours/link types, macros and some of the minor views.
The two applications have completely separate licences.