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.

All the negativity is public and algorithmically boosted, all the positivity is in private chats. And this is increasingly the case over time. I sincerely hope for maximum enshittification in the form of AI slop rendering these misery machines discredited in the public's mind so the spell is broken.

How's reddit a closed platform? It's superbly indexed by Google. In fact searching site:reddit.com often greatly improve my results.