Capaldi as the Master was an April Fools Joke the idiotically got spread around as a "rumor." The idea that a lifelong Who fan like Mr. Capaldi would agree to the lie (and give up a chance to play a dream role) is ridiculous.
Not even having Pink as the first POC Doctor would make up for such a lame deception.
*EDIT: Okay not to harp on this further, but seriously any piece of theorizing or fan-fiction that people spread around as a "rumor" makes my blood boil (especially when alleged "news" sits run them as a story). And in this case, it's just completely nonsensical. It would imply that Matt Smith regenerated into the "real" Doctor off-screen, and that his final speech in "Time of the Doctor"-- ie. one of only two truly honest and beautiful scenes in that episode-- would have been a ruse by The Master? Ugh. So stupid.