I don’t know what you’d consider proof. I don’t have screenshots, as I don’t take them for no reason, and I wasn’t part of the beta myself. But as the Discord communication during the NDA period was located in and moderated by a community separate from Fatshark’s official channels, despite FS employees taking part in it I’d consider it a third-party effort. You could probably still find the posts containing the invite and other info on Reddit (and possibly the official Discord channel’s message history and Steam), but as it’s been more than a month since that, it’s a chore to seek it out (and my google-fu isn’t good enough). FS does a lot of its communication through less official channels, and a few of the devs are certainly more active (or at least noisy) in Reddit than they are in here. I’ve also gotten the impression that the few large unofficial Discord groups have a large dev presence, even if I don’t know about their activity there.
Besides, I brought this up as a comment on the original post’s mention of private Discord channels, just to point out that FS has their own channel, too; that its contents were (during the time when communication was open and afterwards) available to everyone; and that the closed communication (again, during the time when NDA was in effect) in Discord was done through an external community and, as I understood it, initiated by that community’s actives, even if it was with FS’s approval and cooperation.
I don’t fully condone that kind of communication closure, spreading the channels widely, or limiting the official communications to the forums either. I’m trying to say that for that part of it, FS isn’t to be blamed, as they (to the best of my knowledge) weren’t the ones to initiate that. I’d love to hear a clarification on this in either direction.