I have Garmin Pilot +IFR; and can assure you that is pure supposition not based in fact; the subscription belongs to the device and can only be used by building a SD card with the proper serial number for use in that device. There is no way to "use" files within Garmin Pilot.
You can't display charts on a GTN6xx series (not even a 'Charts' button on the interface); only on the GTN7xx so I'm not sure why they sell flight charts for the 6xx. Very disturbing that Garmin wants about $100 of your money for charts when you can't view them on a GTN6xxx; at least I've not...
