It's not how heredity works and Leareth was confused about that before, but he also remembers a lot of Quendi thinking about orcs – maybe they're all believing it on the word of the Valar?
...but his mind is catching on something else. 'But we're not evil gamesplaying people' is...off. It's not how he expects Maitimo to think, and so it isn't how he should speak. Maybe it's because he's still in shock from how fast everything went wrong – that's actually pretty plausible – but maybe it isn't that. They're in Melkor's domain, where he has absolute power. If the real Maitimo was captured, then maybe Melkor can create an illusion of him. Imitate him, but not quite perfectly.
(Maybe the real Maitimo wasn't captured... But that's a fleeting hope, how could he not have been, he and Vanyel would've been even more helpless against Melkor's surprise attack than Leareth was.)
If he had Thoughtsensing he could check... Then again, maybe there's a very good reason they're not allowing him even that single, harmless, passive use of magic.
A very fast chain of reasoning. Maybe he's wrong - it's a gamble - if he bets wrong in one direction he kills his friends, but if he bets wrong in the other, then...a lot more than two other people die. And there are other worlds. He isn't the only chance for Velgarth.
Maybe his immortality works even when all the rest is blocked. Maybe not. He won't know until later – but he didn't know the first time, either.
I am sorry. In either scenario. He doesn't know and he can't know, he's been outmaneuvred by a god, and he has to make a choice anyway.
The motion to call down a Final Strike is blocked entirely. Which is irritating. It means he can't even do any damage on his way out.
"No," he says, flatly. "I do not believe our goals are aligned - though certainly you have every reason to attempt to convince me they are, which means that I trust your attempts even less. I do not wish to help you. I suppose you can kill me if you wish but I will not do the job for you."