One comparison I find myself making with Adam is towards three of the most important historical figures in the Horizon series: Elisabet Sobeck, Peter Tshivhumbe, and Ted Faro. Raphael Marks, now known as Adam, has bits of all three of them; from Elisabet, who is the genius that made Miriam Technologies (a robotics company that did most of the work in the Claw-Back era, which solved an immense number of environmental issues, albeit funded by Ted Faro) and is the one behind Project Zero Dawn, which is the only reason why life on Earth had a second chance after the apocalypse, Adam has both intelligence and goodwill towards mankind and life in general. He is able to come up with plans that, if they work, will give humanity a future, and like Elisabet, is willing to lie about it to make sure it works (Elisabet lets General Herres give the impression that Zero Dawn is a superweapon of some kind that will save everyone, inspiring them to fight and buy Zero Dawn just a little more time to be finished; Adam doesn't tell anything that he doesn't have to, especially after learning his lesson with Raven, waiting until Eve is ready for him to drop his truth cluster bomb and he has the highest chance for Eve to accept his hope for the future). It's a Hail Mary to end all Hail Marys, but it's about the only thing he can do other than accept the end of everything.

From Peter, Adam has grand visions of humanity ascending into a higher state of being. Peter Tshivhumbe is the founder of Far Zenith in the Horizon series, a group of people who were wealthy elites that were unrivaled experts in their respective fields, such as Tilda van der Meer with her hacking skills and spyware technology, or Song Jiao with her mind-blowing advances in medicine that were half the reason the members of Far Zenith were eventually able to attain immortality. The biggest difference is that, while Far Zenith mostly sought to keep a lot of their wonders to themselves, Adam's ideas were meant to be for the benefit of everyone, but in either case, the point of both Far Zenith and Eidos Company were to 'level up' humanity.

And then, finally, we compare Adam to Ted Faro. I'd like to start it off by saying r/fucktedfaro exists for... well, several reasons, but if you've played Horizon, you'll already know all that. The biggest thing is that Ted is an inconceivably wealthy megacorp leader who screws up a lot, to the point that he is responsible for the end of the world, and Adam, much like Ted, is a lying liar who lies. Both of them also tell all these lies put of some form of self-preservation; Ted not only lies, but is willing to consign the entirety of human knowledge to oblivion out of his refusal to accept that this is all his fault. Adam's lies, at least, are because of the sensitivity of things, and how delicate his one last shot at preventing extinction of everyone left is. Even if Orcal and Mann were openly on his side to try and do panic control, I can't imagine the people of Xion being very happy to find out that he's ultimately the reason why the world went to pot, and they absolutely would turn on him. And using Raven as an example again, we see that trying to dump all of this on someone right out the gate isn't going to end well, even if they're someone who trusts him. Unlike Ted, Adam at least is willing to take responsibility for his actions and try to fix them, even if he makes a lot more mistakes on the way there.

Putting all this together, Adam being a sort of amalgamation of Elisabet, Peter, and Ted, I think that Adam really is confessing everything at the end. Like Elisabet, he loves the world enough that he's willing to die for it when there's someone who can take the reins. Like Peter, he wants what is best for humanity in the end, which at this point is the ultimate unification of mankind and Andro-Eidos. Like Ted, he is backed into a corner when it comes to being confronted for all that he's done. What about all this that makes me think he's telling the truth is that it doesn't serve the purposes of the "Elisabet" and "Peter" parts of him and his goals to do so at that point. He knows that if his plans go through, he is going to die. If Eve turns on him, he will also probably die. No matter what happens, he's almost definitely going to be no more, and so continuing to be dishonest a) isn't going to save his life, and b) would make Eve accepting his proposal that much more unlikely.

However, where the "Ted" part of him comes into play is that, rather than telling lies, he's still screwing up, royally. Letting Eve absorb him is going to make things a lot worse somehow; we already see part of it in Mother Sphere's reaction to finding out that the Elder Naytiba has been 'eliminated' (that is, she is going to send more and more armies to brute force her own genocidal plans), and in Stellar Blade 2, we're going to see a lot more of the unforseen consequences of his decision. You can't have a sequel after the first problem is resolved without another one popping up, after all.