There's a general "Linux" one and a "SysAdmin" one. But yeah, no specific Arch one.
Also, there's a version of that subreddit using GPT-2, if you're interested, seems to provide way cooler results compared to the one you linked which uses Markov Chains.
https://reddit.com/r/SubSimulatorGPT2
Edit: https://www.reddit.com/r/SubredditSimulator/ (doesn't appear to have an archlinux one though, based on the bot naming scheme)