I fly with Garmin 76S and 76CSX. Like all Garmin models they cannot be
hacked.

Yes they can. Garmin tends to use industry standard processors and
components in their units so there's no reason why they couldnt be
hacked if there was something in it for someone.They use flash memory
for their code, they have serial uploaders to facilitate flashing new
updates. The fact that they haven't been hacked is simply because
there is no reason to hack them!