I'm not sure exactly why, but common sense would state that, with treatment-resistant participants, you already know "1000 things that don't work", and so you've narrowed the search space.
If the way they're doing this treatment involves hitting the subject with magnetic stimulation in "all possible parts of the search space" that might potentially solve the problem, then if you've already half-treated a subject (stimulated one entire side of their brain, say) and it didn't work, that means both that they're "treatment-resistant" and that the second treatment should only focus on the other half of the brain.
If the way they're doing this treatment involves hitting the subject with magnetic stimulation in "all possible parts of the search space" that might potentially solve the problem, then if you've already half-treated a subject (stimulated one entire side of their brain, say) and it didn't work, that means both that they're "treatment-resistant" and that the second treatment should only focus on the other half of the brain.