If it doesn't have networking (which it doesn't) one would imagine the security attack surface is significantly reduced, and pretty much (outside of insane NSA style radio-transmitter-in-your-cable style attacks) requires physical access, at which point many (most?) machines are sort of hosed anyways, unless you've got things like SED/FIPS, etc.
What stops someone from creating files that exploit memory safety or other security issues, then sending them to a TempleOS user? Sure, they have to load the file into the system, but if they're using the OS it'd be safe to assume there's a way to do this.
Exploiting a non-networked machine isn't very useful. There's no practical way to get any data back, to use it as a bot worker, or to send spam, so what's the point?
Smuggle data out in whatever format the user saves data? Corrupt calculations so the user ends up destroying expensive equipment? It's the same issue as attacking airgapped networks.
