Ok this may sound weird..... but this morning I clicked on two episodes and watched them from the front page which seemed to be konohamaru's dream sequence, which shows him learning the rasengan at different times of naruto's grown during shippuden turns out later on it was meant to be Tenten's...
Who knows? It is Tobi-rama's jutsu after all, maybe he knows how to release it. Also how can you underestimate the 1st when you've never seen him fight properly? He did stand up to madara's EMS and 'win' after all.