Was George Orwell an Optimist?
Abdullah Ibrahim - Water from an Ancient Well
Here's the bit that I think is confusing you:
Attacker makes his attack declaration.
The attacker reveals that the attack is by an artillery unit that COULD HAVE led the attack if we made it to step 5 of the attack procedure.
Okay, "the artillery unit DOES NOT HAVE to move." (This is the Exception to having to move into the vacated local.)
When the defender retreats before combat, you're not obligated to reveal that the attacking unit was artillery. The only time you must reveal the attacking unit in this case is if the attack threat was cavalry moving by road. As the attacker, you may reveal artillery that was in attack position, but the reason you would do so would be to avoid the obligation to advance.
Your lone artillery can certainly advance after the defender's retreat before combat. But as your attacking unit was in the approach, it could have been any type of unit and if you want to advance you shouldn't enlighten him.
 I had to answer a phone call in the midst of typing that reply - should have checked to see if it had been answered in the interim before posting. (Great answers, guys.)