Regarding existential types: yes, there's an intimate relationship between existential quantification and OOP. But you can distinguish the difference between OO languages and (non-OO languages with existential types) in the type theory. That's what's important. I think the raw empirical nature of my academic community consensus test is far preferable to the author's epistemological argument.

The question is whether OOP has meaning. I think if the PL community can be empirically demonstrated to have enough consensus that languages are reliably categorized as OO or non-OO, then we can safely say the term has meaning regardless of any armchair philosophy to the contrary.

Edit: And I think (hope) the author of the article would agree my test is sufficient. He states: When I say that there’s no such thing as OO, I mean, more precisely, that there exists some abstraction (or several) that is referred to as “object-oriented”, but that this abstraction has no actual referent in reality. It is an abstraction that is made in error. It is not necessary, and serves no cognitive purpose. It is “not even false”..

If OO has enough meaning to an important subgroup of people that they can use it with more-or-less consensus, then the claim that it has no referent in reality is clearly empirically denied, at least for that subgroup. And if that subgroup happens to be a large chunk of the PL community, then I think that's an important enough subgroup to settle the larger question.