To be specific, you can't use one account because they'll get mad if you ever refer to there being others using the same one; you also can't use multiple accounts because that's "one person using more than one account" in their eyes. Cute little catch-22 there.

A month ago, they banned someone else for being plural too: http://05command.wikidot.com/forum/t-12642041/nixie

It's pretty clear to us, speaking as someones who have previously spent about two years around (and in the good graces of) the staff, that they just don't believe that pluralfolk are legitimate and think we must be just making it up for attention, or perhaps too dangerously insane to be around their community. It's really garbage and we have little respect for them anymore.

In the linked incident log, a prominent admin states their opinions pretty bluntly:

"I'm going to simply state this and there will be no argument to it we will entertain. If you occupy the same physical body, chatstaff will be treating you as a single person within our chat. What you do on your own time is up to you. This is non-negotiable. We have voted and decided that no further references [to being plural] will be tolerated."

Since this statement is based on ideas that are just factually not accurate and indicates extreme hostility to further conversation, there's plainly nothing more for us to do here. We'll be deleting our Wikidot account and no longer recommend this community as a safe haven to anyone, plural or otherwise.

They can all rot in hell.

More broadly speaking, this seems to arise from a long-standing frustration with newbies mistaking the site for a roleplay community, and so they have a sort of knee-jerk reaction to crack down on anyone saying things like "I am not human" or otherwise indicating neuroatypicality in a context that is not strictly medical, because they assume that any such is an attempt at childish roleplaying.