Since I'm re-reading it at the moment, the statistical impossibility of Anathem is intentional - the Incanters / Rhetors are working overtime to drag everyone into a timeline / narrative where things work out for Arbre, and the one they end up with isn't even consistent, hence the confusion with ground support teams as to how many of them are even alive on the black mirror, and why Jad kills their radio, to give the I & R more wiggle room without the comms from the ground locking them into a timeline.
It's entirely possible I & R have actually been working on this endgame for decades, when you think about all the things that have to work out the way they do for the ending - they need the civilians to be led by someone Math-adjacent (the Warden of Heaven) to let them be okay with Maths having power again, getting an ITA on the team required that ITA (Sammann) to have a shared connection with someone on the team (Cord, which implies they needed a timeline where she was working at the machine shop, which happened years and years ago), etc. etc.