Not to be the fly in the ointment, but you can't really just add those up and expect that number to be accurate if we're trying to look at unique users within the fediverse. If I had to hazard a guess, a not-so-insignificant chunk of those probably overlap (i.e. users who have made multiple accounts across several instances). I have made an account across lemmy.world, lemmy.ml, kbin and fedia just as backups in case one instance fails. I might be an extreme case having 4, but pretty sure it's becoming increasingly common for people to have at least 2 accounts (1 on a different instance).
And this is why things like pirating are not only acceptable but necessary. When companies lock services behind paywalls for products we should legally own, we are left with no recourse but to obtain the services we are owed illegally.