Is there any explaination for this error other than "This World is currently full. Please wait until an opening is available and try again."

There's no reason why this error should even come up with the queue system. You could be in queue and this error will pop up halfway through and you're kicked out. Going by the wording by the error, you will never be able to login considering that even during the content drough last expansion, there was a daily queue of 10-40 players every day, unless you play at 3 AM or something.

Alternative explanation is that they're just slapping this error on anyone above X people in queue so they dont have to deal with them, which seems to be the case since people with this error cannot even access the queue system.

It's really frustrating that this launch is inundated with so many errors (3001,2002, launcher errors, "please wait and login later" error, random disconnects after duties) that it makes it a pain just to even login. And this isn't even the main launch date, just early access.