I am saying, on the other hand, that no one has a duty to engage in that discussion, and people, or sites, or activist groups, should not be criticized as close-minded or echo-chambered for refusing to engage in any particular discussion.

So where does the decision point lie? Can we point to a pattern of refusing to engage in honest discussion as a marker of close-minded-ness?

no one should be criticized as close-minded or echo-chambered for declining to participate in a series of discussions that, odds are, will be essentially identical.

Those words there, "no one should be criticized for..." are really, really dangerous. I can think of very, very few things that can follow those words and be a true statement.

The problem here is that, while I have no doubt your beliefs are sincere and well meaning, they are actively harmful. There is no such thing as a healthy environment where nobody feels they should engage anyone with a dissenting view as it's a "waste of time".

I argue that by posting your words on the internet for everyone to read, you've opened up yourself to criticism (free speech and all that), and by denying that criticism repeatedly or having that repeated denial be part of your group's gestalt , that group is then open to legitimate criticism for being a close-minded echo chamber.

In other words, the choices as an activist group are to either make it a habit to engage with dissenters, and be perceived as reasonable and logical, or don't, and be perceived as an echo chamber?

There is no substantive difference between a lack of conflicting opinion because of active censorship, and a lack of conflicting opinion because your group is a frequent troll target and therefore nobody will engage with a dissenter.

This is the definition of echo chamber. Here it is straight from Wiki:

In media, an echo chamber is a situation in which information, ideas, or beliefs are amplified or reinforced by transmission and repetition inside an "enclosed" system, where different or competing views are censored, disallowed or otherwise underrepresented.

Echo chambers are as echo chambers do. The reason doesn't enter into it, the effect does.

..solely because they decline to engage with some particular subset of activists across the aisle.

Again, there is a huge difference between an individual declaration of "meh, I don't want to deal with you, you are likely a troll" (even though this is terribly insulting to a normal not-troll person and problematic on its own), and your ideological group making it a habit to dismiss criticism in this way.

Given what you've told me, how do you intend to differentiate (or rather, expect anyone rational to differentiate) between a close-minded group that invokes the troll card to deflect criticism, and a not-close-minded group that is trolled to death and has a high false positive rejection rate?

So you're encouraging me to change the subject by discussing my beliefs? You know, the meta level of this conversation is kind of skyrocketing :P

No, that's what the word "aside" denoted. If you don't want to talk about it, fine, just say so, but I was just making an interesting observation that presented itself - your views seem very characteristic.