Not just that. Without supplementation, UV still does nothing. If you feed them insects and give them UV, you're going to have problems and you'll be lucky to even keep them alive long term. Supplements are essential. UV makes no difference either way. Without supplements, UV won't help.
I've never experimented with natural UV (sunlight) being a substitute for dietary supplements and I'm not entirely sure if the supplements are more making up for the difference in natural diet or natural UV (I strongly suspect it's the diet, and if you give them whole lizards or mice etc you don't need supplements or UV), but artificial UV definitely does nothing and I am pretty sure natural UV wouldn't do anything either if they're getting a diet of unsupplemented captive bred insects.
UV is a really big myth in herpetoculture. It's probably useful in dragons in some situations, but I'd never bother with it for snakes, geckoes, skinks, legless lizards, monitors or frogs, and yes, I have extensive experience working with all of them. Turtles aren't something I've worked with as much, but I've raised turtles from eggs to adults with no UV and perfect health, shell/bone structure, etc, and I know of 30+ year old turtles which have never seen UV.
