The sound differences between abs and pbt caps are mostly related to the density/mass of the materials. Pbt is a denser material than abs, so in general pbt keycaps are heavier than abs and therefore sound more muted/lower pitched.

Abs is technically a "softer" material than pbt, but the difference is negligible, especially when compared to the impact mass and density have on acoustics. This is probably what causes many of the misconceptions.

Not sure if it would be too heavy for a beginner's guide, but adding a little blurb about keycap thickness couldn't hurt. With many of the budget keycaps beginners tend to buy, thickness tends to matter the most. Cheap abs keycaps tend to be pretty thin (less mass, higher pitched), and cheap pbt caps are sometimes even blends of both pbt and abs