The original thought-experiment asked us to imagine that all British people suffered from salmon-phobia. This assumes a level of distinction that in real life, we would regard as a fallacy — because in the thought-experiment world, we could truly say that if someone wasn't offended by salmon, that proved they weren't British.

I don't think the original thought experiment would change much if the aliens only hacked 85% of British people chosen at random rather than every single one.