The question obviously was not meant as an epistemological one to which "well, something must exist that thinks" would be a meaningful answer. It was about justifying one's values without an objective point of reference, as perhaps offered by gods. There was really no reason to have Descartes's answer there, because it does not answer the question the game wanted to ask. (By itself it's a pretty pedantic and useless answer to the epistemological question as well, but that is beside the point.)
I disagree. The question by and large refers to "if you've been lied to your whole life, what can you ever be sure of?", where the direct subject of the lies is ~spoilarz~, but it obviously extends to any subject you could apply. It is not, after all, 'what if we can't be assured of ~spoilarz~', but what if we can be assured of *nothing*. You can put here your own existence, your knowledge, your beliefs, your gods, your history, your anything. And Descartes's "I can at least be assured of the fact that I think" hits the nail on the head here, just like it did that originally - the idea behind cogito ergo sum being a sort of a reassurance a man can have the moment he starts questioning everything and is about to go crazy, he can at least be sure of his own ability to cogitate, even if everything around him is nothing but an illusion, some bizarro matrix. It's not about "something must exist that thinks" in the context of the question posed by the game, but "I must exist because I think". "I am not an automaton". "I have not been created by soulmachinery". "Even if I've been lied to all my life, at least I can consider these lies".
Plus, if it's justifying one's values without an objective point of reference - if
everything is likely to be fake, how is the very
fact of thinking
right now not the only objective truth available at this very moment that could be used as this point of reference?
(finally some actual criticism I can respond to, thanks)