Now either x exists or it does not.I follow the first leg of the argument. If there are non-existent objects, then existence obviously cannot boil down to instantiation. That is very clear. It is the second leg which makes no sense. The argument seems to be that some philosopher is blogging, only because a blogging philosopher exists. Therefore the existence of a blogging philosopher cannot boil down to some philosopher being a blogger.
Suppose it does not. Then we have instantiation without existence. If so, then existence cannot be instantiation. For example, let C be the concept winged horse and let x be Pegasus. The latter instantiates the former since Pegasus is a winged horse. But Pegasus does not exist. So existence cannot be the second-level property of instantiation if we allow nonexistent objects to serve as instances of concepts.
Now suppose that x exists. Then the theory is circular: it presupposes and does not eliminate first-level existence. The concept blogging philosopher is instantiated by me, but only because I possess first-level existence. One cannot coherently maintain that my existence consists in my instantiating that concept or any concept for the simple reason that (first-level) existence is what makes it possible for me to instantiate any concept in the first place.
One might as well argue that this meat being lamb cannot boil down to this being mutton, for it is lamb because it is mutton. Therefore the definition 'mutton is lamb' is circular. But that argument is blatantly bad.