Directors don't usually compose in music. They 'direct' compositions and scores made by other people.
A few composers also direct.
In the cases of games, this may mean they head the music and sound team, and do the final finishing edits and touches. But it rarely is anything as grand as the work of the Composer itself.
By the way Mark Seibert is a nice guy, he answers emails. If you want to know more about his involvement in SQ4 you should ask him yourself! His email is on his site.
Also, another thing to look out for. Ken Williams often gets top billing in many of the Sierra games, as a game producer. But his actual game involvement was pretty slim!
Same goes for Sierra's mid-90's "Creative Director", Bill Davis who often got third highest billing many of the Adventure games of the 90's. Again, his actual role on each game was next to nothing!
http://sierra.wikia.com/wiki/Bill_Davis