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