I'm not saying that anything with an occupant load of 50 or more is a Group A--I'm saying that anything that meets the description of a Group A per Section 303.1 and has an occupant load of 50 or more must be classified as a Group A. For example, a large open office with an occupant load of 100 is not classified as Group A because it is not "for the gathering of persons for purposes such as civic, social or religious functions; recreation, food or drink consumption or awaiting transportation." However, these "Paint Nite" events are purposefully intended to be social and/or recreational--thus, conforming to the description for a Group A. And, as you stated above, "where not classified as a Group A occupancy" is mentioned within the listing of "Training and skill development"; thus, they are indicating that these spaces must be classified as a Group A when they conform to the conditions of a Group A.
In the list for Group A-3, "Lecture halls" are listed, so adult education (above 12th grade) must be considered when classifying for Group A, even though it is not specifically listed as such under Group A as it is for Group B. The "Training and skills development" is added to primarily distinguish these type of education programs from those classified as Group E, especially when the occupants could be in the 12th grade or below.
You can try to classify it as a Group B, but don't be surprised when a comment comes back to change the classification.