A common pattern among Reddit alternatives has been that their initial population has been some particular group that has been driven out of polite company for some reason. Voat had the Incels and Trump supporters, for example. Lemmy happens to have been started by authoritarian fans. The difference is that Lemmy is open and finally the "normal" Redditors are coming. So I expect them to flood out the niche folk soon enough.
Why would he have announed the counteroffensive? It would only have helped the Russians. He's declaring it now because it is obvious. And because the offensive is making gains as you say.