......They are basic dressage movements because they are what a basic dressage test asks for. They are also the basics of good riding. Because they are both one and the same thing.
Except they are not "one and the same thing".
W/T/C/circles/serpentines done by a jumper in a forward seat would not score well on a dressage test. W/T/C/circles/serpentines done by a western rider in a western saddle wouldn't do so great, either, unless done by a WP rider - but then the horse would move in a way that no dressage judge would approve of.
When I cantered my Appy this morning, I used a western saddle, a forward-like seat but not exactly, loose reins and a rope halter. My western instructors would have been horrified, because I was leaning forward. My weight was almost all in my thighs and stirrups. My horse was extended, going just short of a gallop. I neck reined him. Was it good, basic riding?
Look at my signature. I was in balance with my horse. My gelding was making good time, ears forward, head down (for him). My thighs were glued to the saddle. When he thought about slowing, I whispered "Come on, boy!" and he went back into rhythm. When I settled back a little and took some slack from the reins, he slowed to a trot. Settled further in, and he walked. Said "Whoa", and he stopped.
There is nothing wrong with having a horse and rider W/T/C/circles/serpentines in a basic dressage test. If given by dressage judges, I'd expect them to evaluate based on the rider and horse using an approach that would set them up for success as they continue further on in dressage. But they are NOT basic dressage movements, because lots of horses and riders perform W/T/C/circles/serpentines in a very non-dressage fashion - and do so appropriately.
The basics of dressage and the basics of good riding are not interchangeable. The basics of dressage set you up for success in dressage. The basics of jumpers set you up for success in jumping. The basics of barrel racing set you up for success in barrel racing. But the proper W/T/C of each will look different.