Does it actually matter that Discord is closed or would the problem still exist if Matrix won and the content contained within was nonetheless still not indexable by search engines and communities were invite only?

Because the fact that Discord is flourishing is I think a direct result of it not being a part of the giant amalgamated omniplatform indexed by search engines. Being shielded from the masses is the point. There aren't megaphones in small private communities where the social dynamics of repeat interactions are in play. People aren't competing to say the most outrageous rage-bait for engagement.