Yeah, I'm on the side of everyone else, reptile carpet is pretty great, whereas coconut husk has the risk of impaction if you have noticed some accidental consumption of it in particular...
That said, the guy at the pet store wasn't necessarily /wrong/ exactly. A lot of "loose" substrates are fine, provided everything else in your husbandry is spot on, however the chances of everything else always being spot on is pretty low. Loose substrate plus even slight dehydration (which could come just from skipping out on a salad or being a picky eater/drinker) is a recipe for disaster, and hardly any experienced reptile keeper is going to risk it.
If your main concern is it being boring or dull, I would focus more on getting new hides and places to climb for your dragon. They are semi-arborial, and Guacemole (my dragon) loves his different logs and hides! I have about 5 and I rotate which 2-3 are in his tank every week or so. Really seems to liven him up a little. Another thing I do is sew my own reptile enclosure liners out of fleece. I use remnants, but it comes in a huge veriety of colors and patterns that can make the enclosure more appealing. You need to make ABSOLUTELY CERTAIN that what you're using doesn't pill or shed fibers and hem it (easy to do - if you have any friends with a sewing machine it will take them less than five minutes) but once it's done it's super easy to clean (because the wetness from the poop doesn't "soak" into it, just use paper towel for cleanup) and you can even through it into a washing machine to deep-clean it (I use a detergent with no scents or dyes for this, but you probably don't even need a detergent).
If you're specifically wanting to provide digging/borrowing opportunities, look into getting a smaller container you can put inside his main enclosure. I still wouldn't recommend it, but doing this reduced the risk of your beardie eating the substrate on accident.
Hope this is helpful!
-Ellentomologist