Actually, he lied (now that's something new... but at that moment he probably decided to become the Hybrid, see below) as the transcript of "Hell Bent" shows (emphasis mine):
ASHILDR: Sometimes you do. It's always the way with things we'd rather
forget. You remember now, though, don't you? Tell me, Doctor, who is
the Hybrid? Who threatens all of Time and Space?
DOCTOR: Oh, but that's easy. That's very, very easy. The Hybrid is you.
ASHILDR [on scanner]: I'm human, with a little bit of Mire inside me.
The Hybrid is supposed to be half Time Lord, half Dalek.
DOCTOR [on scanner]: No, it isn't.
DOCTOR: The actual prophecy specifies only two warrior races. The
Daleks and the Time Lords have made assumptions, of course. And they
would. Humans and the Mire, both warrior races. It fits perfectly.
ASHILDR: It's an interesting theory.
DOCTOR: Do you have a better one?
ASHILDR: By your own reasoning, why couldn't the Hybrid be half Time
Lord, half human? Tell me, Doctor, I've always wondered. You're a Time
Lord, you're a high-born Gallifreyan. Why is it you spend so much time
on Earth? (Because in the Movie, the Master worked out that the
Doctor's really half-human - on his mother's side.)
your best theory? I'm the Hybrid? I ran away from Gallifrey because I
was afraid of myself? That doesn't make any sense.
ASHILDR: It makes
perfect sense, and you know it. Am I right? Is it true?
DOCTOR: Does it matter?
ASHILDR: No. Because I have a better theory.
ASHILDR: What if the Hybrid wasn't one person, but two?
ASHILDR: A dangerous combination of a passionate and
powerful Time Lord and a young woman so very similar to him.
ASHILDR [on scanner]: Companions who are willing to push each other to
DOCTOR [on scanner]: She's my friend. She's just my friend.
ASHILDR [on scanner]: How did you meet her?
ASHILDR: Missy. The Master. The lover of chaos, who
wants you to love it, too. She's quite the matchmaker.
DOCTOR: Clara's my friend.
ASHILDR: I know. And you're willing to risk all of Time and
Space because you miss her. One wonders what the pair of you will get
up to next.
DOCTOR: Nothing. Nothing at all. I know I went too far. I
get it. That's why I'm doing what I'm doing.
ASHILDR: And what would that be?
DOCTOR: I'm taking her back to Earth. Somewhere safe,
somewhere out of the way. I'm going to wipe her memory of every last
detail of me.
So the gist of it is, the Doctor didn't actually know until Ashildr rubbed his nose in the answer. And the Hybrid is not him
alone, but him and Clara together.
His claim to be the Hybrid is rather based on the prophecy he quotes,
The Hybrid destined to conquer Gallifrey and stand in its ruins...
Well, after what the Timelords did to him what they "just"1 did he basically lets them (or at least the confession dial) know that now he has decided to become the Hybrid and tear Gallifrey down. Obviously, after having had some time to think about it, he didn't but rather set in motion the events of "Hell Bent" which in turn did
threaten all of time and space because the Doctor wanted to save Clara and thus almost fulfilling the prophecy.