I don't think that's completely true. A good developer must have a good knowledge of the stack he's working on. So he should be capable of managing that stack, shall the necessity arise.
However, it's obviously better to separate matters and offload management/administration tasks to a separate team/person. Thus, a good developer in a good company (which has a separate sysadmin roles) indeed can't really replace good sysadmin because the latter has niche practical knowledge on handling various situations (especially emergencies) quickly.
Nonetheless, one can be both a good developer and a good sysadmin at the same time.