Still confused... Easy Gaited Horses gives the impression that a horse can't rack without being somewhat hollow. The only easy gait that can be rounded at all is the foxtrot.
This is true. The lateral gaits require some measure of "hollow back."
The centered gaits will have a mostly neutral back.
The diagonal gaits can have a measure of "bascule."
So if you have a horse with native gait that is lateral you can ride that gait (hollow back and all) IF
you recognize what you've got and vary the gaits to allow the horse to use other muscles and give the back a chance to move in other ways. I think Lee has said this in a number of different ways.
So, if you've got an L4 horse then you've got to do a bit more "square walking" and canter work to keep the back supple and working than somebody riding a running walk. Of course that running walk horse should do some canter work, too!
With a trotter you live with the rule that "a trot is a trot is a trot." With gaited horse you live with the rule that every horse has a native way of going that can be easily influenced and altered by human action. Some alterations are Good Things (i.e.,mixing up the gaits) and others are Bad Things (i.e., using devices or practices to force a gait).
There's a lot of "nuance" to working with gaited horses.