These metal oxide particles are harder than the DCC head and its protection.
Normal tape heads consist of big chunks of ferromagnetic metal where a voltage gets induced, they should be harder than most particles and if a little bit is missing, it is not a big deal.
DCC-heads are based on delicate thin-film magnetoresitive heads which means the resistence changes. They consist of very small structures, certainly softer than the particles. The protective film out of a special material helps a little, but it can’t be blocking it. Philips even assumed a limited lifetime for DCC-tapes which we were thankfully not able to confirm. Take a look at those microscope pictures for a better understanding of the damage: Technics RS-DC8 buzzing sound on analogue playback - #8 by didyman