Philosopher notoriously disagree about nearly everything, including the essence of their field and subfields. Nevertheless, no one even lightly familiar with philosophical ontology would say this. I must believe you have misread it.
You may be confusing the broad field of philosophical ontology with a very specific family of arguments known as the "ontological argument" for the existence of god." and which are commonly dated to 11th century Europe. They are not generally considered within the field of philosophical ontology. Moreover, although occassional new versions of this type of argument pop up, the general view is that the argument strategy is fundamedntally flawed. For a basic summary see
http://plato.stanford.edu/entries/ontological-arguments/, but please do not be confused. This is no about the field of ontology, or arguments within that field, but only about a specific thread of philosophical-theological arguments that happen to have the word "ontological" in their common label.