The Southeastern website says:
That statement is true, but it perhaps should be clearer that it doesn't by any means cover all National Rail journeys.
There are many journeys that are not available on e-ticket because the journey crosses London and TfL do not accept barcoded tickets. These journeys may not be made available on smartcards by the fare setter as long distance operators take the view that journeys that will generally require seat reservations are not suitable for smartcard fulfilment. For your journey from Saltash to Folkestone the fare setter is Great Western so this is out of the hands of Southeastern.
So how has it been allowed that there are now no ticket fulfilment options for an ever increasing number of journeys as TOCs phase out orange card tickets in favour of paper QR tickets? Basically if you start your journey at any unstaffed station without a TVM, then there will be no fulfilment options.
As for SE's dinosaur approach to E and mobile tickets, I know of someone who was penalty fared at Charing Cross with an outward portion of a super off peak day return from St Albans to London Terminals for which she changed platforms at London Bridge. She was at first told by the revenue staff it's not permitted to to travel on a booking confirmation (that's what revenue thought the Eticket was)
Even after she showed them booking confirmation email and parts of the ticket that clearly state all London Terminals it's valid too and also the note that says "show this ticket on a device or print it" they were still not having any of it.
Obviously the penalty fare got cancelled pretty sharply but I'd hate to think how many people have fallen foul of this.
Also now that Southern have fully rolled out Eticket, there's a lot of mobile apps that will only offer e ticket fulfilment if it's one of the options available now. Problem with that is there is now quite a few journeys like Marsh link stations to say Gatwick Airport for which travel via Ashford/Tonbridge (route Edenbridge) is fully valid and permitted, and is often also the quickest route. What is SE going to do about this?