It's been a while since I've been at that scene, but the ultimate resource back then was The MUD Connector.
In particular, it has an advanced search page you can use to filter muds with a specific number of players online on average; for example, performing a search with the "100+ players" checkbox selected shows 23 MUDs.
- That search page is a bit clunky, for example some options are selected by checkboxes instead of radio buttons even when only one option is allowed
- I believe the site's database depends mostly - if not entirely - on the feedback from the MUDs themselves. So if a MUD's admin don't update it often you may find that it has less or more players than advertised, and it's possible some popular MUDs might not be listed at all.
Despite these two caveats, this site was pretty accurate and popular back when I last used it... admittedly that was like a decade ago, but I hope it is still up-to-date and would help you too.
I understand you already checked that site; I guess that when using the MUD connector, it's important to pay careful attention to the "updated" date for the MUD. For example, of these 23 MUDs mentioned above, only around 5 have been updated in the last 18 months; so I would skip the others and check these directly. Indeed, one of those 5, called the 3 Kingdoms, offers an browser-accessible "who list" which, at the time of writing this, lists 90 players currently online.