[Whoops, premature “Send”, this is the real thing.]
To be clear, I did not mean to say that making a site accessible requires it to be accessible with text based browsers, strong ARIA support is (as you already said). However, accessibility is more than being nice to screen readers, it is being nice to any (sensible) form of accessing the site, including non-/limited-JS agents (be it a browser, braille device, bot, et al.).
While making Discourse editable by non-JS agents is certainly not in scope for the project (and rightly so, at least for the time being), making it readable and navigatible for those agents is relatively straightforward and IMHO important for this software (though still not easy).
Sidenote: There are more reasons for JS not being available for an agent than missing/limited support or the user having disabled the feature, e.g. when there is a Network error or Internet access is poor (as in many many places even in the Western hemisphere! e.g. I’m currently browsing ultra-slow on my phone since I went past my monthly budget) or a proxy/ISP strips or manipulates the code (as it happens in many corporate environments and has happened with mobile providers!). Sometimes the user is just not in control of the network/his environment, and this will still be true in 10 years. At least reading & navigating should be possible in those situations.