Interesting! From their website it looks like you must become a "school" (about $4000 USD) and that gives you access to the material? I guess theoretically you can be a school of one for teaching a single child.
You can also work with specific institutions that will allow your kid to take the IB tests if you’re in a jurisdiction that doesn’t require you to legally be a school if you’re home-educating (as you’ve said, Utah does not).