They are correct. When you save a waypoint it is stored on the DF memory. Always a good idea after you have an inventory of WPs stored to save them to an SD card. The Navionics card is write protected to prevent corruption of the data stored on it. So get a blank SD card and you can save your waypoints, screenshots, and tracks if you wish. Makes it really nice if you upgrade to another DF. You can import the info from your SD card to the new DF.
IF .
If you changes makes of DF you can still do this but the conversion process to make it compatible with a different brand is cumbersome.