I think it's more to do with the fact that Noblessner has old industrial buildings to break up the monotony of new buildings. The same goes for Rotermanni which is rather uniform in the height of the buildings but doesn't feel unnatural.I would argue there's still a lot of leeway in that rule of thumb with regards to floor heights and roof styles. As it stands now Kalaranna kvartal feels to me like it could at least use an extra 2-3 metres in places, if just to break some of the monotony.
Matter of fact most of the new seaside developments suffer from very uniform building height rules IMHO. Porto Franco and its surrounding buildings developments feel unnatural as a result. Noblessner is somewhat better.
I think it's just very difficult to create a natural-feeling built-up environment that's 100% new, especially if you are developing a huge area at the same time. The new development area around Helsinki's Western harbour looks rather lifeless as well IMO, probably for the same reason.