You are right abot the FP Act definition going out and you are also right about the potential for building owners to leave it to the occupiers, (esp. if they are employers).
However, each employer will have to ensure that their 'relevant persons' can not only escape safely from their particular unit but to a place of ultimate safety. Therefore an employer on the 5th floor must ensure that once out of the his/her unit the relevant people are safe all the way down to the ground.
In practical terms this may appear to be overly onerous but the definition of responsible person also includes 'any person who has control of the premises' and finally, 'the owner, where the person in control does not have control...'
It would be a very hard case for an owner/managing agent to argue in Court that they had no responsibility for common areas. or in the example you mention, common systems if those areas or systems impact on safety of others.