Based on the dialog in that same episode: a vessel traveling at warp "seems to disappear." And picard describes it as "what any good helmsman would have done."
The operative trick in the Picard Maneuver is to momentarily avoid detection with a short-distance warp hop, giving you just enough time to get into firing position and the bad guys insufficient time to obtain a new firing solution. We can derive from this that a starship's sensors cannot really track a vessel moving at FTL velocity; the ship instantly outruns its own reflection (whatever EM signature the sensors are using to image it) and for all intents and purposes doesn't have a real location in time and space until it stops.
So even if the Ferengi had realized what was happening, there'd be a moment where they have to release the weapons lock on the first image and get the computer to treat the second image as a new target, lock on and fire. Stargazer drops out of warp with only a single target and can more quickly get a firing solution, giving it the initiative. All of this, again, depends on the sensors not being able to track a ship moving faster than light, and more importantly, being dependent on radiation that moves at light speed.