"it can't actually do anything that other many languages can't do too"
I personally think it's really difficult to scale systems without loosing messages. Having the OTP available in GenServer and GenEvent in Elixir is amazing and provides a nice way of creating your own servers. A good example of these primitives in use is the Redis Driver; it uses two processes - one to send messages to Redis and one to Receive them. This means that essentially all messages going to redis are performed in a set and forget way. Most other clients (even in erlang) block and wait for a response from redis.
Now I am not saying that it's impossible to write this stuff well in other languages/virtual machines but I am saying that Erlang/Elixir/the OTP makes it easier.
"Yes, it's a great default to be shared-nothing"
I'm not certain you could call elixir shared nothing. It's simply wrong; I'd look at the Elixir primitive/server called Agent which a specific construct of sharing state.
"it is not an unreasonable response to point out that there are plenty of other web servers that have similar levels of isolation"
I'm not certain you understand what the OTP is giving you over a webserver or an operating system process, it's much more about having the ability to have a message queue system baked in and easy to use that happens to be supervised and has the ability to be moved to different machines transparently.
"This is bad advocacy, and I'd really suggest that Erlangers stop trying to defend this point"
You are arguing against a straw man for me here. Maybe the original poster is a bit about this but I couldn't care less about the number of processes. I'm much more concerned that Elixir allows me to think about building better and more scalable software with a lot of edge cases solved for me. Once you get that every section of your program can be thought of a server which you can send messages to and this is super efficient, you don't need to wait for anything to block anymore it really is a better model.
"Erlang will not have some sort of unique claim to being able to run lots of such processes"
It can definitely be done in other languages but it's much harder.
"I wouldn't be surprised that nginx's isolation is more tested than Erlang itself's."
It's never been about isolation, it's about not blocking and sending messages instead. And getting supervision and robustness and fault tolerance and easy of use. Yes you could have 300 rabbit queues in front of every part of your program but I'd say it's easier and more scalable to just use Elixir/Erlang/OTP.
I've programmed in Erlang for somewhere around 6 years. Which really makes things like "I'm not certain you understand what the OTP is giving you" rather silly. 
You're falling into the exact trap I was talking about, which is implicitly assuming there's some sort of magic sauce that only Erlang brings to you. You also are so busy disagreeing that you didn't notice that I already said pretty much everything you said. (What is it about Erlangers explaining back my own points to me, only angrily as if I didn't get the points I just made? I get this more from Erlang than any other community.) I'm not kidding about Erlang being a milestone language. But it's not magic.
"You are arguing against a straw man for me here."
What I'm arguing against is Joe Armstrong's attempt to redefine terms to suit his advocacy. If that's a "straw man", take it up with him, not me.
: See also http://www.jerf.org/iri/post/2930 . And for goodness' sake, please do not reply with another laundry list of the points I already made in that post about how that isn't exactly what Erlang has back to me as if I didn't make them already. I've already been through that. (You know the differences because that post tells you exactly what they are.)
I'm glad you clearly get the OTP but that is definitely not clear to me from your previous response. Good luck.