Correct; Of course, even if the mapped boundary is a straight line, my analysis presupposes that the wall itself is a straight line all the way: it is perhaps possible (though unlikely) that the internal party wall is thicker than the external gable wall - I suppose it would depend on construction methods used.
It is also worth pointing out that boundary maps are not always drawn with precision, and that the position adopted by people will usually depend on context and circumstances. For example:
1. Your friend goes to knock garden wall, neighbour says 'don't touch it, it's a party wall and I like it'
2. Your friend finds a crack in the external gable wall, asks neighbour to pay half the cost of fixing; neighbour says 'it's your wall, nothing to do with me'
It would be perfectly possible for the neighbour to form either of these views in good faith (but not both simultaneously....)