Ah, Reddark is only tracking subreddits that said they were participating, the ones that listed themselves in the Modcoord threads. There are far more subreddits that exist than the 8,829 they're tracking.
Just one important distinction: not a quarter of all subreddits, but a quarter of the subreddits that said they would go private for at least 2 days are still private.
That would be against the terms of service for using the API and a surefire way to get your app removed from whatever storefront you have it listed on as soon as Reddit complains.
It's the number of users logging in and hitting those servers that's the main issue. 44,000 users is a lot for one instance to be pushing content to. I wouldn't say it's just aggregating Lemmy content either though, there are plenty of popular communities on kbin as well, including this one.