Yes, the primary difficulty with multithreading isn't implementing the threading itself, it's keeping those threads from interfering with one another in terrible and unexpected ways. If you build software with threading in mind from the start, it can be done pretty cleanly, but at this point in DF's development it's likely more trouble than it is worth. The amount of effort that would go into multithreading DF would almost certainly be better spent simply optimizing algorithms in stead.
Multithreading is a frequent suggestion on the forums, and this is almost always the response it gets. I would suggest starting another topic if one would like to explore the idea and it's possible implications further.
More on-topic, I'm curious to hear the next few devlog entries. I can only imagine the kinds of bugs that are showing up in fortress mode right now. The way the new combat mechanics work for a fortress guard should be interesting - I expect that it will be much less wise to send a single legendary dwarf out to greet invaders in this coming version. I would also like to know if brawling (in stead of just one-sided tantruming) is now something that can happen in an unhappy fort. Dwarves have had listed grudges for a long time, and it would be fun to see that come up in play.