I'm sorry, but this information/advice/characterization is pretty horrible... It's like saying you can keep your ball python in a tub that's half the size it should be or that we can lock our dogs in the bathrooms all day. Most rhac keepers I know - unless they are commercially breeding - tend to offer their adult gargs a lot more than a measily little 12x12x18. There's a big difference between survival and thriving though. So I guess if you only want it to survive, then sure, cramp it up in a 12x12x18. If you want it to thrive then put it in something larger.
I put baby cresteds, in pairs, in 12x12x12s and never have had issues with them finding their food. And I know someone that got a fairly small gargoyle gecko back at an expo in February and put him straight into a 12x12x18 without issue as well. A lot of people tend to baby their rhacs a lot more than they should be... I know a couple breeders that have large set-ups (around 60 gals in one case) naturally planted and with water features and they grow out their babies in that without losing any.