Maybe they thinked about xmpp open to registration like standalone. Like the scribe that everyone can sign-up and "take" the username from people at disroot.
The xmpp addresses will be for your account @disroot only so noone can register your username at getgoogleoff.me or disr.it to take it for xmpp like people can "take" your user at git scribe and nexcloud i guess on disroot
I even registred at scribe some hours ago so no one takes hate from me lol
Not at all it was just a demo for the project at https://github.com/rystaf/mlmym and you can use it. Some instances added the old. to use like https://old.lemmy.world/ or https://old.lemmy.sdf.org/ but it is up to the administrator of the instance to add it.
Maybe a request for a custom css theme at lemmy.ml will do it