Generally true, but not always practical. Sometimes the course owner, designer, or someone in authority is not on DGCR. Then, it would certainly be good to ask them before proceding. Sometimes that person can't be found. A lot of little 9 holers in small towns wouldn't be listed at all if some random disc golfer hadn't taken it upon himself to add it---and we're better off for it.
Yeah, I've added to courses that were put in in city parks. I found out about them because a family member sent me an article in the news paper about their opening.
Several months later when I returned to my home state, they were still not listed here. I played them, photographed them and listed them.
I think a simple rule applies. Provate property, let the owner do it or ask. Public park, go ahead and list it when it's in the ground.