Maybe “not on other boards than those using an ATmega32u4”?
For example the LilyPad Arduino USB uses that MCU too. And, for example, Badgerboard tells one to select that LilyPad Arduino USB board in the IDE, for which the TTN library compiles fine then. I don’t know if it fully works as expected; that’s a bit hard to tell with the little documentation Badgerboard provides.