Anyone considering whether they may use a Travelcard on the HS1 route between St Pancras and Stratford would do well, alongside the zonal map, to consult our old friend, National Rail Enquiries, as surely this (and not "The Manual") provides the definitive source of customer information for all passenger rail services on the National Rail network.
http://www.nationalrail.co.uk/times_fares/ticket_types/46587.aspx#crossing_Travelcards
HS1 is surely a National Rail service and the maps seem to show it runs inside, not outside, the zones.
http://www.nationalrail.co.uk/times_fares/ticket_types/46587.aspx#crossing_Travelcards
Travelcards can be issued in a range of zonal combinations and subject to the zones purchased, you can use your Travelcard on:
Tube, DLR and National Rail services (but not Heathrow Express or Heathrow Connect services between Hayes & Harlington and Heathrow)
All London Bus services (except some special services and excursions) regardless of the zones for which the ticket is issued
London Tramlink services where the Travelcard is valid for travel in Zone 3, 4, 5 or 6 or any combination of these zones.
HS1 is surely a National Rail service and the maps seem to show it runs inside, not outside, the zones.