Pretty sure they are storing the name history periodically for each user.
You'd have to find another site which logs usernames because Steam users are able to delete it from their account. Doubt Steam exposes deleted names.
That site is pretty good but their username (history) search is bugged, it won't show all matches for a query.
You could always just buy premium on their patreon, then you'll be able to see the full names from the past.
