What you say is correct. We can't know it definitely isn't true. However, what I am trying to address here are the arguments that have been advanced by Bostrom and others for why we should believe it is true. We can't know anything about the nature of the superstructures in which the universe is embedded, if there are such structures. Which means nothing can definitively be ruled out, including, as I say, the possibilty that we are the dreams of a purple troll in Xanadu. But this does not give us any reason for believing a specific scenario, such as that we are in a computer simulation, which is certainly implausible, for the reasons I have outlined.