In this context, it sounds like he's using "stretch goal" to mean something like what Jim Collins calls a BHAG - Big Hairy Audacious Goal[1]. I like the concept, and need to make a note to myself to sit down and define what our BHAG at Fogbeam is. We've been so heads-down working on tactical stuff lately, there hasn't been much time to stop, back up, and think about BHAGs.
[1]: http://en.wikipedia.org/wiki/Big_Hairy_Audacious_Goal