I don't think anyone thinks the simulation argument is any reason to expect a singularity soon so I'm not sure why he bothers with it.
Perhaps Stross is treating Singularitarianism as a package of beliefs. Since people who talk about the Singularity also tend to talk about the Simulation Argument, the package of beliefs must contain the belief that we are living in a simulation. Thus any critique of the belief package must address the question of whether we live in a simulation.
I periodically get email from folks who, having read "Accelerando", assume I am some kind of fire-breathing extropian zealot who believes in the imminence of the singularity, the uploading of the libertarians, and the rapture of the nerds. I find this mildly distressing, and so I think it's time to set the record straight and say what I really think.
Short version: Santa Claus doesn't exist.
- Charles Stross, Three arguments against the singularity, 2011-06-22
EDITED TO ADD: don't get your hopes up, this is pretty weak stuff.