I do note with interest that "what I have said is not a lie" is very different from "what I have said is the truth", which makes me wonder what the Ancient one is actually planning.
Ah, missed this question.
Remember that while her divinations beyond that are very imprecise, the Ancient One can scry the timeline in detail up until her death in 2016 - and Age of Ultron is in
2015.
And in this timeline, if Cap hadn't returned then Ultron would have
succeeded in destroying all life on Earth, because the Avengers function
much less effciently without him and it was a damned close call as is.
So when Cap got wormholed, the Ancient One wondered '... why the fuck did the future suddenly just shift to total human extinction in 2015?'. And of course she started looking with the Time Gem, and of course soon enough found out. And between the whole part where the ending of life on Earth is bad
and the fact that the creation of Ultron involved the misuse of an Infinity Stone - something that's actually within Kamar-Taj's purview to go interfere with - she figured to hard derail all that shit.
However, for obvious reasons the Ancient One can't
mention Ultron, for the exact same reason that in canon Dr. Strange did not mention that the 'one chance' in Endgame involved Tony's having to sacrifice himself. And that reason is, if she told him what needed to happen... it wouldn't happen.
So the Ancient One
very carefully timed Cap's return - because she could have gone and fetched him earlier if she wanted - for two things. First off, the timing was such to make sure that Courtney returned with him, both because the Ancient One is human enough to not want to inflict further tragedy on the man if she doesn't have to and because Courtney's joining the Avengers is one of the two things that the Ancient One is derailing the creation of Ultron with. And the other thing is, of course, Tony and Steve actually deepening their friendship... which is the other why Steve's return is timed for
right after Iron Man 3 ends, when Tony Stark is in his most psychologically vulnerable state and at a cusp point where a positive influence can send him in a better direction. (It is
not a coincidence that Steve and Courtney are warped directly into Stark Tower at a time when Tony is there convalescing and on pain meds - Tony's emotional defenses are as far down at that moment as they're going to be at any point in the next several years.)
And as can be seen, it works. Things butterfly enough that not only does Tony actually talk to his friends about things
and Steve doesn't try to hide Bucky's involvement in the Starks' deaths from Tony (that's Courtney's influence right there), but in the process Tony comes to the realization about just how fucking dangerous the Mind Gem really is... so he's not going to plug it into his AI research.
This is why the Ancient One tells the entirely perfect truth about how Cap's return is necessary to keep Thanos from fucking shit up later - which it actually is, so she's not even lying there - but doesn't say a word about averting the creation of Ultron. She doesn't need to.
As Malevola said, mages like the Ancient One do love their secrets and agendas. Even when they're relatively benevolent ones.
And before you ask 'why isn't this in the fic?', it's because I didn't consciously realize the full scope of what the Ancient One was doing until after you asked the question. I mean, I did plan some of the bits in advance (the pain meds, and derailing Civil War), but even I didn't realize how it all fit together until after the question prompted me. Because
yet again, my muse did shit and then didn't tell me about it until later.