This does not undermine the effectiveness of the traditional treatment. Whether or not children with normal feet also wear these special shoes doesn’t tell us anything about the effects of those shoes on arch development. This is just irrelevant.

This does not undermine the effectiveness of the traditional treatment. We’re not interested in the pros and cons of flat feet versus high arches, we’re interested in whether the treatment for flat-footedness actually works. This doesn’t help with that at all.

This does not undermine the effectiveness of the traditional treatment. This doesn’t tell us whether or not there’s any relationship between wearing special shoes and the improvement of flat-footedness, it’s just irrelevant.

This undermines the effectiveness of the traditional treatment by rebutting the assumption that wearing special shoes actually improves arch development. If there’s no correlation between treatment and arch development, we can’t call the treatment effective.

This does not undermine the effectiveness of the traditional treatment. We’re only interested in the correction of flat-footedness, so a fact about non-flat-footed children is totally outside the domain of this question.