I agree that squatting, especially the full squat is a good exercise for the lower back. I also agree that lower back can be a limiting factor in the squat sometimes, especially for folks with a weak back.
HOWEVER, it is not just a lower back exercise, it is a lower body exercise that happens to hit the lower back hard as well. This is like saying that deadlift is a grip exercise and not a lower back exercise for some, since for some, the grip fails earlier than the lower back, or its like saying that bench is a delt exercise and not a pec exercise and so on ...
For heaven's sake, these exercises are known as COMPOUND exercises for a reason - they hit multiple muscle groups. The squat is not lower back OR
lower body, it is lower back AND
Here's the opinion of someone who does know what he is talking about - The Squat routine with Arnold Schwarzenegger