The Sansa differentiates between songs transferred via MTP mode and those transferred via MSC mode. The demo songs are all in the MTP area and thus you won’t see them when looking at the device in MSC mode.
But it is possible, to mount the device in MTP mode also. You just have to install the packages libmtp8, mtpfs and maybe mtp-tools. Now connect your player in MTP mode and do a
(make sure, the directory
/mnt/sansa exists before trying the mount command.)
Now you should have access to the MTP area of your Sansa where you can delete the demo songs or put new files in.