that makes sense about the fluidity.  who knows though, wasn't that book from a while ago? things do change. araki, we know, always tries new things when it comes to jojo.  also take into account what kind of story part 8 is. it's not so straight forward story/plot wise  as a part 4. 
it's a part filled with mystery, many points and small details.  he's now even included a "this thing and this thing, is happening at this time" setup that he has to keep track of. 
now, i don't think he's on a "haha! I've planned everything from the start" kind of mindset but i'm pretty sure, more planing is going into things  then you may think.