They are advertising a enterprise ready solution and it's clearly not. No matter how much hardware you throw at it.
The largest has several tens of millions of users and they are using Global Scale. One customer runs nearly 4 million users on a single cluster.
Nextcloud is a complicated beast, and scaling to tens of thousands of users isn't trivial, but then that is what Nextcloud Enterprise is for. So if you need a large scale installation, no problem - contact our team and we get you up and running.
Finally there are very few orgs worldwide even with a few hundred k employees and any firm would get huge Microsoft discounts at that scale for products like SharePoint (on prem), Exchange (on prem) and Office Server (on prem also)... and oodles of support over a very mature product which Nextcloud is simply not.
Edit: Just so you know we are still a customer of yours with over 3k users but not for long anymore. We tried to work with you for over 4 years now and got absolutely nothing. Neither performance nor features.