I think a combination of young age and being recruited by the Obsidian Order made him make that mistake. He was too young to really know the truth and the Order certainly didn't tell him much, but promised a lot.
Brenok, in spite of not-so-perfect record, wanted a good pilot. This was a turning point for Sabal and a great change and Brenok saw with his own eyes that records don't always have to tell the truth. They are as good as the people who write them.
But yes, Sabal was one unlucky fellow