I have just purchased this ticket as even with NR PRIV this £6 is still cheaper.
What it shows on the actual ticket is
“London Undergrnd Zones 1-6” to “Farringdon.
It looks like its a LU only Cash Fare from looking at TfLs single fare finder. However this might have been overlooked when Crossrail fares were hastily added and then removed.
There are no route restrictions or “Underground Only” as used to be shown on paper tickets.
How ever what is even more interesting is the journey planner and travel itinerary shows you using Heathrow Express.
Heathrow Terminal 5 (Rail Station Only)
Anytime Day Single
Train operator: Multiple operators
Ticket restrictions: This includes all direct services and connecting services recommended by the journey planner