The reason why TSF can't join regular 1.23 servers is because TSF is basically a stand-alone game. TSF features an extra character (Lori), plus some significant overhauls, including expanding the tileset limit (1.23 can handle tilesets of 1020 tiles, TSF four times the size).

Because of the many differences, TSF is not compitable with 1.23. The only compatibility you can speak off is that TSF can play v. 1.23 levels, but that's it.