If they know about certain technical exploits or problems, they include them in the build. So yes, in a way they are included.
I realize the answer is pretty obvious, but can you say for certain that every hotfix from Windows 8 is in 8.1? I'm talking actual proof, not just a generic "oh, it's a newer operating system, of course it has previous updates" statement... I really can't think of any specific hotfix to test though, so meh.