Just one of those early episode oddities that we have to write off, like when Harry Mudd recognizes Spock as a "Vulcanian" in "Mudd's Women." Apparently the implication was supposed to be that the half-human Spock looked somewhat different from a full-blooded Vulcan.
No, the term "Vulcanian" was like how we have Earthling, human, Terran, Earther, etc to describe people from Sol III.