That may be true but it is the first time I have heard that in connection with a handheld. FM Immunity refers to receiving equipment (VOR or LOC), not transmitting equipment.
I wouldn't be surprised if it was just another French 'scheme'. Years ago I was denied approval for the installation of an ICOM in a French microlight, because the radio was purchased in the UK and ostensibly did not meet French standards. This was in spite of producing a letter from ICOM Japan certifying that the UK and French models were identical.
ICOM FRANCE : Produits Radiocommunication : Accessoires Platines et filtres : FL-IFFM1