I can't see why there should be a restriction in text in a forum post - it's not like anything will be looking for executable commands. Perhaps parties with Xenforo knowledge could look into it and see if it's a hard-coded restriction or something in the configuration for this forum in particular, or maybe imposed by the web host.
Either way, it's taken this long to find that I don't see it as advice newbies need, and non-newbies won't be looking at the guide anyway. It makes it bloody difficult to discuss anything about operating systems though.