It's not their theory. it's some radical theory brought up by some desperate ME3 ending haters that's already been debunked. But it's amusing how people insist on bringing it back. Let me make this perfectly clear, never in a million years will the indoc theory have anything to do with the mass effect universe.
By not committing to any endings and ignoring the conclusion of ME3 they've opened the door for any and everything to be canon. Including absurdities such as "It's all a dream". Whatever you want the ending to be, be it RGB, MEHEM, IT, Deception, Refuse, Carebears come in and save the day thru the power of love, the Biotic God 1-hiting all the Reapers, whatever you can imagine is all equally canonical.