Tobi knows he can't beat the sage xd. Anyway, this could have all started with the massacre, Tobi obtains the eyes of an Uchiha with a limited time technique and starts his doctor who routine, that means he could have met Madara, and fought Minato, then lost that to Izanagi. That means he would have waited long and that statement would have made sense, his senju DNA only makes it work enough to use it very few times, maybe the original technique was a once in a lifetime thing :shrug:. I'm pulling rubbish out of my arse now simply because I am as stumped as you, don't take this too seriously.