The Frequency Compensation of LT1683 in a DC-DC Converter
After checking the article and the datasheet of the LT1683, I'd say that Q3 (see image inserted below) doesn't have anything to do with frequency compensation.

According to the datasheet, pin 12 (Vc) of the LT1683 is the output of the error amplifier and the input of the current comparator.  The input part is what is interesting when looking at the Jim Williams article.  The error signal (output) drives pin 12 to ground when a short circuit occurs.  The current comparator then shuts down the LT1683 to prevent damage.

The 2A laser driver uses pin 12 to shutdown the LT1683 during start up.  It does this by pulling pin 12 to ground through Q3.  This lets the the chip start and get operating without passing any kind of transients to the laser. Once the input power to the LT1683 is stable, Q3 opens up and allows the LT1683 to start producing output current.  At that point, the 10nF capacitor on the collector of Q3 becomes all the frequency compensation you get in that circuit. 

From the need to do this, I would guess that the LT1683 doesn't regulate its output very well while the (external) powersupply is starting up.  Mr. Williams designed the laser driver so as to keep transients from destroying what is probably an expensive part -  a quick look around turns up smaller lasers that draw less than 1A for over $150 each.



Circuit under discussion from the Jim Williams article: