IBF stands for "Inlet Barrier Filter"
https://www.aero-access.com/products...tion-kits.html
As the IBF is an add-on, the aircraft can fly without it pretty easily....so no need to have it......
As you already quoted the manual: If the manual says "Donīt fly", then donīt fly...as easy as this.
If the IBF is not operational, ask the maintenance guys to take it out-then fly without it (if thatīs possible without recertification)...