The scripts/depmod.sh checks whether the path in $DEPMOD is executable and only executes it if that's the case. So, by setting DEPMOD to "/do_not_use_depmod" the destination path doesn't exist _and_ thus isn't executable aswell.
The scripts/depmod.sh checks whether the path in $DEPMOD is executable and only executes it if that's the case. So, by setting DEPMOD to "/do_not_use_depmod" the destination path doesn't exist _and_ thus isn't executable aswell.