John is a full name. It's the English form of
Johann. If anything, it's a nn for
Johannes, not
Jonathan.
Luke also, is the English form of
Lukas.
Lucas is the Latin form.
John and
Luke have very very long usages as full names in English.
However, I do not like most nicknames as full names.
Bobbie and
Johnny and
Charlie are without a doubt nicknames and not full names.