From what I read in that topic, existentialism is similar in some ways to buddhism regarding it's conclusions, but very far apart regarding the ways to reach those conclusions and what they actually mean by them.
https://www.quora.com/What-is-the-diffe ... of-reality
In some ways existentialism is a little like mysticism in that it strips away layers of assumptions to see things "as they are." The conclusions may be different, but to read the famous park bench scene in Sartre's La Nausée is to encounter a mystical apprehension of unadorned reality. That unadorned reality does exist outside and beyond the self...but at the same time the self can dissolve into it, as just one more component of the gnarled, brutal mess of existence. And that's precisely why Sartre incurs nausea in that apprehension: the hugeness, the lostness, the utter annihilation of significance of the self in that moment, as it is so obviously just an arbitrary part of the whole - like a tangle of roots or a clump of dirt - that the disorientation is overwhelming.
So we have "life is meaningless" that is a conclusion shared by buddhism and we also have "ego has to be annihilated" and the mergence of the self within the big mess of existence. This is an idea different than buddhism, but it's not necessarily hinduist monism as ToVicent said. It is only standard existentialism.
So how can we make this look like buddhism, while being the same existentialist stuff with all of it's tenets ? We just claim Buddha said all of this too but people misinterpreted him for 2000 years because of the 3life interpreation of pattica that actually should have been 1life. Then we say that who ever reads and agrees with this existentialist stuff becomes a sotapanna. And then we say that who ever merges the ego with the rest of existence, destroying the boundaries between internal and external (as the other topic in general discussion is debating) attains nibbana.
It's so standard existentialist stuff only with a buddhist label, same as "secular buddhism". And of course we have the belief that there is a self, cause how else can we have this mergence between internal and external ? We even have the most fundamental Buddha teaching, repeated countless times throughout 10.000 pag of sutta pitakka totally ignored cause we don't really need buddhism, we need existentialism + the pleasure of considering ourselves sotapannas.
How exactly is Nanananda different than existentalism when it comes to fundamental tenets ? It's the same stuff. No wonder this is called "existentialism buddhism".