They get a few "true believer" followers, give them special privileges like beta access (this case), special arcane knowledge (see Urbit), or even special standing within the community (also Urbit, although many other languages where the true believers are given authority over community spaces like discord/mailing list/irc etc.).
I don't associate in these spaces because I find the people especially toxic. Usually they are high drama because the focus isn't around technical matters but instead around the cult leader and the drama that surrounds him, defending/attacking his decisions, rationalizing his whims, and toeing the line.
Like this thread, where a large proportion is discussion about Blow as a personality rather than the technical merit of his work. He wants it that way, not so say that his work doesn't have technical merit, but that he'd rather we be talking about him.
Sadly, there exists a breed of developer that is manipulative, obnoxious, and loves to waste time/denigrate someone building something. Relatively few people are genuinely interested (like the OP) in helping to develop the thing, test builds, etc. Most just want to make contributions for their Github profile (assuming OSS) or exercise their internal demons by projecting their insecurities onto someone else.
From all of the JB content I've seen/read, this is a rough approximation of his position. It's far less stressful to just work on the idea in relative isolation until it's ready (by whatever standard) than to deal with the random chaos of letting anyone and everyone in.
This [1] is worth listening to (suspending cynicism) to get at the "why" (my editorialization, not JB).
Personally, I wish more people working on stuff were like this. It makes me far more likely to adopt it when it is ready because I can trust that the appropriate time was put in to building it.
If you consider a small team working on this, developing the language seriously, earnestly, but as a means to an end on the side, I can totally see why they think it may be the best approach to develop the language fully internally. It's an iterative develop-as-you-go approach, you're writing a highly specific opinionated tool for your niche.
So maybe it's best to simply wait until engine + game are done, and they can (depending on the game's success) really devote focus and time on polishing language and compiler up, stabilizing a version 1.0 if you will, and "package" it in an appropriate manner.
Plus: they don't seem to be in the "promote a language for the language's sake" game; it doesn't seem to be about finding the perfect release date, with shiny mascot + discord server + full fledged stdlib + full documentation from day one, to then "hire" redditors and youtubers to spread the word and have an armada of newbie programmers use it to write games... they seem to much rather see it as creating a professional tool aimed at professional programmers, particularly in the domain of high performance compiled languages, particularly for games. People they are targeting will evaluate the language thoroughly when it's out, whether that's in 2019, 2025 or 2028. And whether they are top 10 in some popularity contest or not, I just don't think they're playing by such metrics. The right people will check it out once it's out, I'm sure. And whether such a language will be used or not, will probably, hopefully even, not depend on finding the most hyped point in time to release it.
All the "public advertisement" he's done was a few early presentations of some ideas and then ... just live streaming his work