I think it's just worded that way. They've discussed it as an add-on in other places.
I'd read it more as "maybe they have you try on the headset as part of the fitting process" and it won't work for you without the correct inserts on hand. Or maybe just that because you need the inserts to use the device, they want to be sure they have them on hand if you end up having to book significantly in advance so you aren't wasting your time and an appointment slot.