This isn't 100% required. If you remove your rear bumper and chop down the third brake light you can get it on the stock carrier with a little bit of pushing and getting a few threads on the lugs by hand, then tighten them down. This will pull the tires snugly into the isolators.
I've been running mine like this for almost 10 months with no issues.
If you want ease of installation/removal of the tire and don't wish to remove your bumper or chop your third brake light, then you'll need a spacer.