The author's main observation is that with a non-functional approach one has to avoid any kind of intra-frame pollution of object data by keeping a list of all changes then applying all of those changes atomically as a final step. This doesn't seem like it has any obvious drawbacks, the simulation algorithm defines all objects updating at once, so it's not unnatural in an OO-world to have this "update step".

The Author calls this universal update step a "fix" and then goes on to say that a functional approach would avoid it, but in reality never explains why a problem exists in the first place.