It take issue with that stat. If you go to all the PDSes and ask for all the accounts, you will reach a total several million lower. This count does not seem to account for takedowns and self-deletions. Even taking those into account, you still don't reach the numbers this site shows

Also, I have 5 accounts, so "users" is also inaccurate

If you want to reproduce my calculation, my tooling is here: https://github.com/blebbit/atmunge

More charts here that cover DAU,WAU,MAU

https://bskycharts.edavis.dev/edavis.dev/index.html