No Yngwie does not use pure forearm. Almost nobody does. One example of pure forearm is the EVH tremolo technique. But there are not many players who use this as their main picking motion.
The Volcano chapter is old and incorrect and we have thought about removing it many times. I probably will eventually! But ironically people found it helpful so we kept it up there.
Anyway yes almost every player who uses ‘forearm’ really uses a combination of wrist and forearm. Sometimes even more wrist than arm.
The recent broadcast we did on using compound picking motions is on this topic specifically and tried to demonstrate what the motion looks like when you involve different amounts of each joint. It’s a complicated topic but I think the simple solution is to understand how to recognize when both joints are moving, and generally what it looks like when it’s more wrist, or more arm.
The reason this can be helpful is because if you use different combinations at different times or for different phrases, and you don’t know you are doing this, it can be really confusing. Your technique can feel different all the time and this makes it harder to focus on one combination and get better at it.