While there is an element of truth in your main question (whether combat required "better trained troops" vs. "less trained ones," the thesis you read seems rather suspicious.

The best comparison for your argument is that of a longbowman vs. an arquebusier. An arguebusier usually took only a few weeks to train, especially as accuracy at the time was far less important than rate of fire. By comparson, a good bowman would often need to be trained for a far longer time, and it was an ongoing experience, as well as strength (to pull the bowstring to fire it at the appropriate range and angle) and endurance (sustainable rate of fire).

The main advantage firearms gave were threefold. Firstly, they were simple to use in combat, allowing one to raise levies and arm them quickly. Secondly, a volley would create a loud sound and smoke, causing confusion in the enemy lines. The most important bit, however, is that they were cheap. Standing armies didn't exist for the most part around the advent of the arquebus, partly because they were so expensive. For the most part, troops were raised by feudal levies, which required a certain commitment to feudalist principles. The rise of the musket allowed countries to slowly begin maintaining a standing army, thus reducing their reliance on feudal lords. Over time, this allowed monarchs to centralize more power underneath them, as they were able to maintain a force on hand, while also reducing their obligations to the lords (although there were exceptions, such as in Poland, where the lords gained more power, probably due to the Polish reliance on heavy cavalry, which remained a very elite group among early modern militaries).

So while I would agree it reduced the need for "highly trained warriors," but it didn't do away with them, and while it took power away from feudal lords, more often than not it gave the power to the monarchs rather than to the common people.

Sources:

Jones, The Art of War in the Western World