He’s just 100% wrong. (@SpruceTexas689 stop typing stuff like that if you’re clueless). There’s quite a lot of multithreading in the sim already.
It is however often ‘limited by main thread’. The ‘main thread’ is the thread that runs the job that keeps all the others synced up.
Saying the sim is single core only is categorically wrong (try running it on a single core, and enjoy your single digit FPS).