Cold process soap does not automatically need glycerin to be added because glycerin is already produced during saponification.
When oils and lye react to form soap, glycerin is also created. In this kind of soap, it remains present in the bar rather than needing to be introduced as a separate essential ingredient.
Why adding more can change the bar
Additional glycerin can make an ordinary soap softer and more prone to absorbing moisture. So adding glycerin simply because it is considered desirable can alter a characteristic you may have worked to achieve: a firm, durable bar.
The workshop contrasts this with some commercial soap manufacturing, where glycerin may be removed. That helps explain why handmade cold process soap should not automatically be approached as though glycerin were missing.
The larger formulation lesson
This question illustrates a principle that applies far beyond glycerin: before adding an ingredient, first ask whether the soap already contains it or whether the process itself creates it.
A good formulation is not a collection of as many desirable-sounding ingredients as possible. Every addition has consequences. If an ingredient is already naturally present, adding more may change hardness, moisture behaviour or the overall character of the bar without solving an actual problem. The useful takeaway is simple: glycerin is already part of cold process soap through the soap-making reaction, so it should not be treated as an automatically necessary extra.
