From another forum, about the same video OP posted.
Like I said in the opening post and in another post after that, I don't really agree with Musk's take on this. I only posted the vid to show that this is a topic that people known as smart and serious do discuss.
As for why the poster you quoted thinks Musk is wrong, I actually have some issues with that, too. If we are living in a simulation, the simulation doesn't really have to replicate reality, so whether or not technology gets to a point where it can replicate base reality is not really relevant. From what I understood Musk to be saying, his argument isn't really dependent on the existence of a technological singularity, just the idea that the possibility of one suggests that one version of reality supersedes all others and that means it's likely we are living in one of the simulations.