This isn't true, we have no idea what technology SE are using, we know that we are moving away from open seamless zones to old school loaded zones.
Loaded zones are no more than massive instances, so it is completely possible to invite players from other worlds into a specific zone. (you could have zone instructions to stop players from other servers moving between zones) for example you spawn in Bluefog area and when you try to leave it kicks you back to your own server.
It's actually possible to make a system with individual channels like Rokien suggested and it could just dump your character in that zone, when that zone is full to maximum it would spawn a new channel. (that's a truly worldless server, although I think we are probably a few years away from that)
