They do what they want to do. It's all a question of how low the frequency has to be, deflection of the floor, etc. It also depends on how they measure the IIC, so how low you get in the data you look at. For info, to put some perspective on numbers:
"Impact Insulation Class (or IIC) (or IIC Rating) is an integer-number rating of how well a building floor attenuates impact sounds, such as footsteps. A larger number means more attenuation. The scale, like the decibel scale for sound, is logarithmic. The IIC is derived from ASTM method E989, which in turn uses a tapping machine specified in ASTM method E492."
The IIC number is derived from sound attenuation values tested at sixteen standard frequencies from 100 to 3150 Hz. Unfortunately, "real world" footstep noise is also generated at frequencies below 100 Hz, so the IIC value may not accurately describe the complete noise attenuation profile of a floor."
Natural frequency of a studio floated floor is usually close to 6Hz. If you don't manage the air gap you'll get in trouble but under 100Hz. With a natural frequency at 20Hz instead of 6Hz, you won't see a difference at 100Hz. You will at 60Hz. See my point?
I do remember they recommend even sealing the floor with caulk or something like that.
I disagree with that, for the reasons mentioned earlier, especially for lighter floors with small air spaces between the 2 floors. It WILL make your natural frequency go over 20Hz to maybe even 30Hz, just from air stiffness.
So, I stand my ground...
You'd have 120mm of air space to play with we'd be discussing some 60mm Rockwool in there.
BTW I don't see where they mention the natural frequency of their floor. I don't think I've ever seen that number either. It's likely written somewhere though.
I have given you advice about how to manage your problem, but you don't have to follow that advice... Do what you want.