You could do that without steam just by clicking through the UAC prompt.
The parent is describing an attack which does not require the administrative user to do anything except install Steam. Any other user who can logon to that machine could use this privilege escalation exploit to access the administrative user's files
However, this is not necessarily the case, so I yield the point.
As a particular problematic scenario, consider a school: Steam cannot be used in educational settings because of this issue.