Gut sounds are not indicative of stomach acid amounts, but simply of the intestines moving food/feces on through. An empty stomach seems to echo and magnify that sound more, but does not mean a horse (or person) is empty when you do hear gut noises. A certain number of sounds is considered normal, while too few or too many can signal a problem.
However, during the treatment, if you you fed your horse different (either kind of food or hay, amount changed or frequency) that could alter how his tummy was working, and it could have been that that helped his ulcers, especially if you also changed back to the prior feeding regime after the ulcerguard treatment was over.
More fiber (Hay, hay pellets or cubes and beet pulp) and less concetrates (grains) can keep the belly full longer and provide good gut motility, and cut down on ulcers from the fact that they utilize the acid produced, and buffer it, but can de-stress the horse, as his natural food is high fiber, which requires more chewing and less time fretting about when the next load of sweet feed arrives. I"m making generalizations here, and not in any way judging your feed plan, either.