I think it's interesting that the article linked to above uses ChNN as an example that Dzogchen is Buddhist because refuge, bodhicitta and dedication of merit are essential to all DC practices.
I think it's interesting because the link was posted in response to the statement that Dzogchen in not Buddhist, which is something that ChNN says in at least several of his books.
Just quickly flipping through the first few pages of "Dzogchen: The self perfected state" I see multiple quotes such as:
Although...Dzogchen was introduced and spread around the country by the two great traditions of Buddhism and Bon, Dzogchen itself should not be classified as a religious or philosophical tradition.
-Adriano Clemente Editor's Introduction
ChNN states, on p.27
For example, those who already have a certain familiarity with Tibetan culture might think that to practice Dzogchen you have to convert to either Buddhism or Bon. because Dzogchen has been spread through these two religions. This shows how limited our way of thinking is.
He talks more in that chapter on this and makes the point, by way of example, that a Catholic priest can absolutely practice Dzogchen without giving up his role. In other areas he has said that a Christian or Hindu can practice Dzogchen.
In other places (that I can't reference off the top of my head) he talks about Dzogchen being the ninth yana confuses people because they think that it is like the other yanas only higher, or that you must practice the first 8 before and/or in addition to practicing anti yoga.
All that being said, as referenced in the article, ChNN teaches traditionally (well, from one perspective at least) and clearly shows and teaches great respect for Tibet and Tibetan culture.
I draw no firm conclusions, and I think that the article made a good point and that a seminar called "Dzogchen without Buddhism" sound questionable at best, but...
I think the disagreement evolves around semantics.
If by Buddhism it means social and cultural conditioning,or tribal affiliation to a religious institution,then the answer is certainly no. One does not need to be Buddhist to practise Dzogchen.
On the other hand,if it means means and methods(i.e. a path) to libration from samsara,then yes,Dzogchen is "Buddhism",as stated in the paragraph from the same blog post by Malcolm I had quoted earlier:
"Like all Buddhist paths, Dzogchen sets out to solve the questions of what constitutes suffering, the cause of suffering, and how we free ourselves from that suffering. It may be objected that the answer to that question is quite different from how it is answered in Hinayana and Mahayana, and in the lower tantras — nevertheless, the fact that it engages these questions in a Buddhist context shows that the context of Dzogchen is entirely Buddhist. In fact, there are numerous Dzogchen tantras that review the whole of the non-Buddhist and Buddhist tenet systems in ascending order from non-Buddhist materialists, Samkhya, Vedanta, and so on; then Vaibhashika, Sautrantika, Pratyekabudha, Cittamatra, Madhyamaka and so on."