It is clear to me that the script is that of an experienced writer. I would
not call him/her professional, but semi-professional at least.
Thus it is also clear that this is not the first thing (s)he wrote in this
script. He must have used this script before and often. (I know Greek quite
well but still cannot write very good in it and it looks not very regular.)
What does this mean? The writer wrote this script often. Therefore it is
quite improbable that a difficult code system is needed to create it. He
wrote many things in this script. It must therefore be some language, either
an artificial or a natural one that is dead now.