This is just straight misinformation. If you go on any of the "big subs" you linked, you'll see that there are far more comments than that per day. For example, in ELI5, by just taking the 5 most commented posts that were posted in the last 24h, they have 700 comments which is more than the peak that Subreddit Stats says they had since July.

Instead, if you go on Subreddit Stats and read the text with the big red font, you'll see the explanation why the API changes have made such a difference:

> Heads up! This data is likely out of date or inaccurate now that Reddit has decided to kill the open ecosystem that existed around Reddit. I don't earn any money from this site, and if my calculations are correct it'd cost me a couple thousand dollars per month with their new API pricing, so yeah. If you can, it's probably worth leaving Reddit for other platforms - especially open-source/federated ones like Lemmy.

My assumption is the maintainer just hasn't edited their scraper at all, and it's now running into lots of rate limiting and missing most new comments and posts. The fact that subscriber growth has remained constant supports that thesis.