So, that was me who said that, and I am all but certain that how you described it is not how it actually functions. So, think of this -- when the data center crashes and people try to log in en masse, you get the same message: "Server is full", and your spot in the queue. Obviously, the server *isn't* full (or at that 100% safe capacity) at this point, nobody is having to log off in order for more people to log on, but there is still a queue -- this makes sense, because obviously if you let everyone log on at exactly the same time, it will probably crash the server again. I'm certain that the normal queues work in much the same way -- it's not that the server is at 100% safe capacity or necessarily reaching it, it's just a matter of balancing the load of current players' activity vs the users trying to log in, and letting those users trickle in gradually so as not to overload the server. Nearly EVERY server has a queue when logging in now, so I would certainly hope that not every single server was near or at that safe 100% capacity.
In terms of "adding more people to a full server", I would honestly rather deal with longer login queues if it meant that my friend could actually play with me on my data center. At least opening up a single new server on Aether would allow for that.