Doesn't mean multithreading it is impossible. Many games with particle simulation use it. Look at Ship Sinking simulator 1.3. That uses particle-based soft-body physics WITH mutlithread.
For me what seems to cause lag for me a lot of times is the air/pressure simulation (lag often goes away as soon as I reset it with =).
The devs of the game could add thread sync to sync up all the threads. If one thread doesnt update, the others will stop until the thread updates.
even with that, higher resolutions would be really laggy