Here are the VSTs he is on about East West my guess are that he bought one of the bundles rather than separately as it is more cost effective.

As the Tyros cannot host VST he is most likely using a computer with one of the many DAWs that are available, thus giving him editing and control beyond any Arranger, and then either using the computer to play the sequence or loading the edited file back into the Tyros.

The usual way when producing an orchestral piece like this, is to get the full orchestral score and then play each section into the sequencer before choosing which parts you wish to play yourself. (The more the merrier) It�s also possible that once they had got the backing to their liking, they converted it into an audio file.

As to the quality of sound, then I started a thread about it here

Bill