I don't know why are users flagged with such user right (the rights log does not show any right changes), but they're a lot of users with 'inactive' permissions enabled. Someone with SQL admin access should remove it from those users. Also, there's no hook in InitialiseSettings nor CommonSettings and the right does not even appear at Special:ListGroupRights.
Yes, it can be added two ways: using UserRights or using DisableAccount (which didn't left any trace in the rights log). I've not checked, but it doesn't look like aawiki does use DisableAccount so I'm not sure where those 'inactive' flags came from.
I'm not sure if DisableAccount was ever used in that wiki, I guess one could browse the history InitialiseSettings.php and see if it was ever enabled there. I think the easy solution would be to query the data of those accounts and see if the password and email fields are empty (which is what DisableAccount was suposed to do, right?) and if they're empty, then yes, we can assume they were disabled.