To be specific, a developer is someone who develops something. A Server Dev. would be someone who develops a server or servers, A Plugin Dev. would be someone who develops a plugin or plugins...
In theoretical terms he is correct when using the word "server dev" when he's actually looking for someone to configure and manage his plugins and/or anything server-side related.
(I wouldn't trust someone who's only developed "a server" or "a plugin", you never know if they're experienced or inexperienced)