In a lot of ways, yes it is. But that's where theory and practice diverge. Could all the current systems of DF be implemented anew in a way that easily scales to however many CPU cores are available? Most likely in some way yes they could be. Could the current systems as implemented be ported to better scale with the number of available CPU cores? Questionable.
Game engines are incredibly complex -- there are usually dozens if not hundreds of systems interacting with each other. Over time, this all ossifies to the point where making changes to one system requires changes to many other systems. This is kind of a fact of life with almost every game engine or codebase that I have encountered, worked with, or heard of. When you start getting into more than a decade of development, there are going to be a lot of tangled systems that are going to be very difficult to pull apart.
Yes, the fundamental problem may be solved for multiple threads, but DF solved it for one thread, and that solution is part of what makes DF fun. Messing with that solution is asking for trouble.