My understanding is that the definition 'International' refers to the status of any particular event, not country it is held in. You have levels from Club, through National to International.......
For example, in period a UK based rally car could have competed in the RAC Rally, which was an International status event, but never in its life left the British Isles. In the FIA's world, it has competed Internationally.
Make sense?
I would add the the requirement for any car applying for HTP to have competed 'Internationally' in period is not set in stone. There are many examples of cars that raced or rallied at a lower level being granted Papers.