You are correct. This is the GM recommended method, primarily to protect the installer if the spring launches itself. Keep a chain on it to give it a leash of sorts.
In my 1971 Pontiac manual, the A-Body description is the opposite, leave the bolts in (loosened to allow bushings to swivel), break the bolt joint at the spindle and lower the control arm until spring flies out. Reverse to reinstall. The GM methods assume soft stock springs and use no compressor at all which I find amazing. If the is on a lift and you lower it down onto one of those tall fixed jack stands I can picture not needing a compressor but I couldn't do it without.
Look at the control arm and I think you will find the jack has more leverage by using the opened ball joint method, but it is less safe. I frigged with using the control arm bolt method leaving the ball joint intact.
The worst part was lifting the arm and getting the bolts holes to line up. I did it alone, with a buddy one of you could be shoving the assembly in or out to get the bolts to line up.