2.4 GHZ is not immune to RFI. However it is much easier for the ignition manufacturer to shield its product at that frequency. Any well made ignition system should cause not RFI on 2.4 GHZ. If you have interference when using 2.4GHZ, I would get rid of that ignition system. It is not well made.
I know someone will say I have this high priced, brand new system and it caused RFI on 2.4GHZ. My answer it is not well made despite its price